Matching hull to engine

[Number19]

Hello everyone - my first post; and I'm a 64 year old newbie to the powerboat sport.

I'm preparing to build my first boat and have chosen Hal Kelly's DU Class MADCAP.

I have the technical decision of pairing this hull with an engine. My first choice is the Mercury Mark A, with a displacement of 44 ci. But under the old APBA classes, the DU class was limited to 40 ci.

I can't afford to buy multiple motors and am wondering whether I should go with the 39.6 ci Mark 55. It has the advantage of weighing only 111 lbs vs 127 lbs. Weight is definitely a consideration because the driver - me - weighs a hefty 230 lbs. The old minimum racing weight for the DU Class was 525 lbs.

Of course there are other considerations and I would appreciate all comments & suggestions.

GH

[Yellowjacket]

The 44 cubic inch motors are legal in D stock.

The weight of these engines in the trim you need to run on that boat are all pretty much the same. That boat is designed for a Mk 55H engine. You might be seeing a difference between electric start or the weight of a different tower configuration, but the 55H is the engine that is listed at 111 pounds, and the other weights could be for the engines with the normal (15 inch) tower. The 55H has a short (12 inch) tower and a smaller streamlined gearfoot that has only direct drive, no neutral or reverse, so it's a lot lighter. To make the boat work right you will need a 55H tower and lower unit. You can't put a normal 15 inch tower on that boat unless you build the transom differently and then the motor is going to be up in the sky.

While the older smaller displacement motors are still out there, they are getting hard to find in good shape and some of the parts (like .015 oversize pistons for the 40 cu in motor) are getting impossible to find. Fortunately there's lots of choices for powerheads. There is essentially no difference in weight between the powerheads, so you can pick what is available, least expensive and most reliable.

I'm setting up my "D" runabout using a Merc 500 powerhead (44 cu in) on the 402xs tower and gearfoot. That tower is the same basic design as the 55H tower (and the gearfoot is the same) but it fits the later Merc 500 bolt pattern. The earlier powerheads have a slightly different bolt pattern than the later powerheads. They changed the bolt pattern for the back two bolts on the Merc powerhead in the early 60's, the later powerhead back two bolts are further back and the early powerheads have the back two bolts closer to the front of the powerhead. There are a few Merc 402xs towers that have both bolt patterns and can use either powerhead but those are pretty rare. The 55H gearfoots show up on ebay generally for something between $900 to $1200 for one in good shape, towers go for $500 or more.

I chose the later 500 (76 and later) powerhead because it's essentially the same as the latere 44xs powerhead. I found a 76 Merc 500 for only $250 and sold the gearfoot for $150, so I only have $100 in a very low time nice running powerhead. The 76 was the first year for the better distributor-less ignition and I didn't want to get stuck doing magnetos that the early powerheads had. The mid-60's powerheads have a electronic ignition that is difficult to find parts for so I wanted to avoid that too.

A complete Mk 55h engine goes for around $2500. If you don't want to spend that much you can build up a motor from parts. The choice is to find a Mk55H tower and gearfoot, and put an early powerhead on it (50's or very early 60's), or find a 402xs tower and put on a later powerhead. Or you can find one of the rare "double bolt pattern" towers and go either way. Another option is to find a 402xs and just use it, if it's going to be a lake racer. The 402xs was unloved because it vibrated more and was slow coming off of corners, but it's probably fine if you just want to run around the lake.

The early 4 cyl powerheads can be run over 6,000 rpm without doing some work on them. The later ones need to have the connecting rod big end clearance opened up if you want to spin them over 6,000 rpm without bearing damage. Not an issue if you aren't racing, just don't spin the later motors too fast without opening up the clearance or you'll burn the big end bearings up. Lots of racers don't mind dorking with the magneto, I didn't want to have to work with them so I elected the later motor, just from a maintenance standpoint.

Much later motors (after 85' or 86' I believe) have a different crank that is set up for oil injection and since I wanted the package to be race legal that bracketed the years that worked for me from 76 to 85. The earlier motors in that range, like the 76 that I found, are very inexpensive and I'm cheap, so that's what worked for me.

[Number19]

There's a lot of information in your reply for me to consider, so I'll just post a few comments at this time. I have a list of "master specifications" for all the Mercury models from the 40's thru the 60's. This information has three models of the Mark 55 Thunderbolt produced from 1956 - 58. All have 39.6 ci displacement : the Mark 55, weighing 111 lbs, the Mark 55E weighing 120 lbs and the Mark 55H weighing 100 lbs.

I've already started shopping around the internet and you're right, for a completely restored engine it looks like I'll be paying about $2,500 and I've already decided that I don't want the strictly racing "H" model. I'll mostly be running on local rivers.

When I first started researching Mercury's, I was considering the 402 from 1977 - '79, but the weight was 142 lbs for the MS model and my running weight was pushing past 550 lbs. and I have no idea how these light boats handle increased weight.

I don't know what the "X" stands for in "XS". I'm guessing the "S" is for the short drive.

My thanks for your input. Whenever I was googling for information, this forum kept coming up, so I just had to register.

GH

[Yellowjacket]

The 402XS is the racing version of the 402. It has the lightweight quicksilver lower unit and gearfoot. The 402 you are looking at (at 142 lbs) most likely has the Merc 500 tower and gearfoot and weighs a ton. The 402XS doesn't weigh as much as a 4 cylinder version. I have one at home and can weigh it and let you know what it weighs. The 402XS is also called the 35SS, so it could be listed in your weight guide as that motor, but it is is lighter than the four cylinder versions.

A couple of additional things to think about;

First is that these boats aren’t just toys. They are seriously fast boats. A D runabout will hit 80 mph at the end of the straights on a 1.3 mile course, and that is propped for acceleration off of the corners. If you prop them taller, and run the motor shallower, the D stock kilo record is 89 mph. For a rig weighing just over 500 lbs, this is serious speed and it’s relatively easy to blow one over or get out of control. The Madcap is an older design, but it’s still a seriously fast boat with a four cylinder Merc on it. A proper racing life jacket, helmet and racing suit is required if you are going to go anywhere near the speed this boat is capable of.

Secondly, your boat was designed to run a quickie lower unit with the prop shaft centerline ¾ of an inch below the planing surface (per ABPA rules). If you just hang fishing motor on it you obviously won’t go nearly as fast, but I have no idea as to how the boat would handle at speed with the motor that deep. Might be fine, but like I said, I have no idea what it’s going to do. Raising up a fishing lower unit that high will likely result in issues with water feed, so you will need to address that if go with a fishing lower unit. More importantly, since the fishing lower unit is a lot bigger in diameter and could provide a lot of lift to the back of the boat, you need to be careful about stability at speed. Depends on how fast you want to go, but you aren’t in the range where Hal Kelly designed the boat, that’s for sure. For that reason, and the fact that it’s just the way Hal Kelly intended it, I’d run the racing lower unit. You can have a lot of fun at the AOMC events and doing vintage shows with the quickie.

Lastly the quicksilver lower unit is somewhat maintenance intensive. Frankly they don’t seal very well and it’s important to keep water out and grease in both the lower unit and the grease packed rear bearing after the water pump that is on the propeller shaft. The lower unit grease needs to be changed frequently and there is a tool you can get to pump grease into the rear bearing, and this makes disassembly less frequent. The rear threads on the lower unit do wear out and it is expensive to rebuild and re-thread the housing, so it pays to grease the rear bearing without disassembling the unit to save wear and tear on the threads. The rear bearing should be greased after each day’s use, and the lower unit oil checked for water after each day’s use. If any water is present, the lube needs to be changed before the bearings or gears are damaged. The gears in these units are highly loaded, and in particular if they aren’t aligned properly will wear out in very short order. I would recommend someone like Ron Thomas (aka the “Foot Doctor”) to rebuild you gear foot. These are relatively simple gearboxes, but they are highly loaded and there are some nuances in building them, and if you don’t do it right, you will end up with an expensive piece of scrap.

The good news is that the parts for the D quickie are actually pretty much available (but not dirt cheap), with the exception of the housing itself. The gears ($450 per set), shafts ($250 each), bearings, seals, impeller, water pump housing and tail cone nut are all available (and not expensive at all) as reproduction parts, so as long as you take care of your unit and keep the water out of it, it can be kept in pretty good shape and give you years of service. Gear sets with a “one off” gear ratio are also available and that also helps improve the gear life compared to the “one to one” gear set that originally came on the lower unit from the factory. There are also a couple of places that can rebuild the threads on the housing or restore the housing, so as long as you don’t break it into pieces it can be kept up. Modern lubricants can help the gear life and there are folks that are looking into better, more modern seals that might improve the sealing, and some people have run these racing lower units for 8 to 10 years without a replacing the gears, so if you take care of them they are fine. I don’t want to paint a horrible picture of the care and feeding of this lower unit. It’s not bad, but it’s not a “run it hard and put it away wet” like a fishing lower unit kind of thing either. It’s better to know up front what it’s going to take to keep this thing maintained than to go off and ruin a unit because you didn’t maintain it correctly. I went through all the same thoughts and made the decision to go with the racing lower unit. If I want to run events like the "Top O Michigan" marathon, or APBA or NBRA I can. If you just go with a fishing lower unit you just have a lake boat and I didn't want that. Depends on how you are going to use it.

[Number19]

...your boat was designed to run a quickie lower unit with the prop shaft centerline ¾ of an inch below the planing surface (per ABPA rules). If you just hang fishing motor on it you obviously won’t go nearly as fast, but I have no idea as to how the boat would handle at speed with the motor that deep. Might be fine, but like I said, I have no idea what it’s going to do. Raising up a fishing lower unit that high will likely result in issues with water feed, so you will need to address that if go with a fishing lower unit. More importantly, since the fishing lower unit is a lot bigger in diameter and could provide a lot of lift to the back of the boat, you need to be careful about stability at speed. Depends on how fast you want to go, but you aren’t in the range where Hal Kelly designed the boat, that’s for sure. For that reason, and the fact that it’s just the way Hal Kelly intended it, I’d run the racing lower unit...If you just go with a fishing lower unit you just have a lake boat and I didn't want that. Depends on how you are going to use it.

In his original write-up "Building MADCAP", Hal Kelly instructed that for any motor other than Quicksilver units, the transom would have to be made 17" high. It might seem a little strange to be thinking "motor" before my boat is even started, but decisions like this are precisely why the hull and the engine must be thought of as a total package.

I'm 64 years old, there are no sanctioned races in my neck of the country, so I'm not going to use my boat for "serious" racing. Part of it is personality. I may not run my boat to its full capacity 80% of the time, but if I get into a little pick-up race out on the river, I want it to "go".

The question I have right now is, If you go with the standard "fishing" motor and replaced the lower unit with a quicksilver unit ( and this is what I'm currently thinking ), is this a reasonable compromise? You would change out the prop for "recreational" use. How would this work?

(edit) I know "H" models and other built just for racing have no neutral or reverse. But would a standard "fishing" motor, fitted with a quicksilver lower unit, have this capability?

[Yellowjacket]

The Mercury "racing" motors, like the 55H, from the factory were always the same powerheads as the "fishing" motors. The APBA allows porting the engines to their specs and the change to carbon reeds, but there is no difference, as delivered by the factory, between the "racing" and the "fishing" powerheads. While the stock racers "blueprint" their engines for what little power it gets them, the only real mod between a "racing" stock motor is that the later powerheads have to have the big ends of the rods clearanced so you can spin them higher. A stock motor makes 50 hp at 6,000 rpm. A built stock motor makes about 55-57 hp peak, but because it can spin higher it's faster around the circle. The real power available isn't that different. Mod motors make a bit more power but the open stacks make too much noise and you'll get arrested if you try to run megaphones in most places that aren't sanctioned events, so what we are talking about here is actually APBA stock versus factory delivered, and there isn't much difference.

The rod clearance was reduced in the 60's to make the engines more quiet and sound smoother at idle and low speeds. Since most normal folks don't run their motors above 6,000 rpm Mercury wanted their engines to be quiet and not sound "raspy" at idle. The racers learned quickly that you could eat an crank and bearings if you spun the tighter motors to 7,000 rpm without clearancing the rods. Noise and a "quality" sounding motor was the rationale and effectively the only difference in the powerheads is the rod clearance. Lots of racers use the allowed carbon fiber reeds, not as much because it provides more power, but if you spin the engine higher and if you fail a reed, the plastic reeds don't do any real damage to the engine. If you swallow a metal reed, it does nasty things to the innards. If you don't spin the motor above 6,000 the stock reeds are fine. If you aren't going to go racing just get a later (76 or later) powerhead like I did, find a 35ss tower and lower unit that matches the bolt pattern, and bolt it on as is and go for it. You can build up an engine like that around 1500 bucks or better and enjoy it. That's what I'd do. You could even paint it white and make it look for all the world like a genuine 55H from 5 feet away, and it would be a lot less maintenance than an earlier motor. With the 44 inch motor you can pull a taller prop (like a 13" pitch) on it without going over 6,000 rpm and believe me you will be able to outrun on top end just about any jetski that comes along. A boat like that would run close to 70 pretty easily with a 12" pitch prop and faster with a 13. Since you are doing it with only 50 hp, it just makes them madder.

[Yellowjacket]

I know "H" models and other built just for racing have no neutral or reverse. But would a standard "fishing" motor, fitted with a quicksilver lower unit, have this capability?

Nope, it's pull the rope and go, but that's not a big deal. Also if you use a later motor you can use electric start and just point it away from the dock and pull the trigger. A small lawn tractor or motorcycle battery will start these just fine. You don't need a big heavy battery and most of the Merc 500's that are cheap are electric start anyway. You need a proper deadman throttle (you can buy them new from Keller for $125 or find a used one for around $50). You just set the throttle a click or two above idle and hit the switch.

[Number19]

Well, I finally got around to understanding your responses and I think I agree. I had just never considered assembling a motor from different models. You provided the information which makes this a viable option and I think this is how I'll proceed. I'm not adverse to saving a few bucks.

[Number19]

If I'm reading you correctly, I need to find a 402xs tower but can use a 55H gearfoot if I can't get the gearfoot with the tower. Finding the tower is going to be the problem, it seems.

[Yellowjacket]

I believe that Jerry Wienandt has a couple of 402xs towers. Get in touch with him at tridentracing@new.rr.com he has a lot of other racing stuff and is thinning his herd. He's also a super guy that knows this stuff inside and out. Do what he says and you won't go wrong. Otherwise troll ebay or keep an eye out here and on Hydroracer forum. Also if you post it in the wanted sections here, Hydroracer or on "John's old Merc site" you may find one. Also there are some threads on here and on hydroracer that cover the subject pretty well.

http://www.boatracingfacts.com/forum...2-44XS-Secrets a lot of info on building and maintaining these powerheads

and this one with info on early towers

http://www.screamandfly.com/showthre...?175380-Mk-55h