View Full Version : motor questions for newbie

08-12-2006, 04:43 PM
hi i am currently building a 14 foot X-1 cab-over hydro and was wondering if you have to use a special motor or if the motor has to be a certain type. i am not going to race just want something to zip around in on our pond, well actualy its like a small lake. and i noticed that hydros have a small fin on the left sponson to aid in turning but since im not going to race and turn in both directions at good speed would i be able to just put a fin on each sponsoon or is that not possible? than you ne info would he helpful to get this newbie safely out on the water...sorry to mention the motor i was gonna use is the old 70 horse evinrude off my 1968 hydroswift out in the back, thank you

08-12-2006, 05:47 PM
You will probably also find that the design of the hydro is insufficient to float a standard motor. Plan on altering the transom and motor mounting height accordingly.

08-12-2006, 06:07 PM
well here is a pic of the motor i plan on using. so by altering the height of the motor mount do you mean making it so less of the motor is in the water? and if so then how much of te lower unit should hang below the hull but the motor will not have the gray plate on it. here is a drawing of the general shape of the hydro and this boat will be 14 feet long and almost 7 feet wide. any help on ne modifications that will need to be made would helpful as i am buildong 2 of these. oh and would putting matine foam in between the transom frame and number 4 frame help give the back more flotation to help support the motor? thank you


08-12-2006, 06:42 PM
Adding foam increases weight and makes the boat lower in the water (not much though because the foam is very light).

I think that boat was designed back when a 120 pound 40 hp was the biggest you could buy. Your 70 weighs about 210 pounds. I suspect your plans call for a 15" transom, you will need to get your motor up to where the top water inlets are even with or above the bottom of the boat; probably closer to 20" from the clamp.

08-12-2006, 06:49 PM
in the plans it specifys a 60-100hp motor be used. if ne one has drawings or diagrams showing what needs to be done that would help. so instead of the transom board being 15" extend it so the height is 20"?

08-12-2006, 07:00 PM
You should extend it so it fits your motor.

08-12-2006, 07:07 PM
ok so the three water inlets on the lower unit should be even with the bottom of the ull or a little bit above it. would it be okay if they were somewhat below the bottom if i did it that way?

08-12-2006, 11:14 PM
as much as id like to im only 17 and have a limited budget. my dad built a hydro almost identical to the one i am building when he was in highschool and that is why i want to do this project. but this is the only motor i have access to so i have no choice but to use it

08-13-2006, 07:36 AM
You will be much happier if you build the newer type boat with the smaller motor - the design is far superior for a modern electric start motor. It shouldn't take much to find someone who wants to trade a 2 cylinder 60 for your 3 cylinder 70.

If your budget is super limited, you should consider a boat like a Minimost and a very small motor over the X-1

08-13-2006, 08:24 AM
i realize there may be better boats out there but i havent found many plans and the x-1 is the best ive found thus far. as for trading someone motors i have no idea where to look. im in spokane washington but i have no clue. and as for a smaller boat and motor im kinda a big guy and dont know if it will handle me im 6' 3" and 220 pounds. as i said before im not gonna race as i dont have the time or funding just want a fast boat to take to lake roosevelt or pondere' which are huge lakes. does anyone know how stable these design onf boats are? and their stability at high speeds? and what about cornering? i have had some input on these subjects from other sources but ne opinions ar info always helps...thanks

08-13-2006, 09:55 AM
just a bit of info on my mini most project you can find the plans for free on the internet www.muskokaseaflea.ca,mine has a wider cockpit than the plans it can fit 2 people I am 200lb and I take friends my size regularly (400lb) I bought a old 18hp for $250 which pushes 2 people about 30mph,the boat took me about 20 hours to build

08-13-2006, 01:20 PM
as much as that sounds cheap and good. its not fast enough for me id just get bored with it then not use it and it would have been a waste of time. plus i want somethin big

08-13-2006, 01:34 PM
better plans on eBay (http://cgi.ebay.com/ebaymotors/52-HYDROPLANE-RUNABOUT-CRUISER-BOAT-PLANS-1_W0QQitemZ250015993105QQcmdZViewItem)

Your X-1 is on there as #11

The guy is a pirate that has downloaded free plans off the internet and burned them onto a CD, but the Aquarod and Aquaracer should be what you have in mind; well able to support a standard motor

08-13-2006, 02:40 PM
I think John Schubert (TART) was involved with the Aquaracer, it was intended to be a J (99ci) racer, it is more than big enough.

08-13-2006, 11:44 PM
so would a tunnel hull be possibly faster and more stable? and would it cost about the same to build? my dad is a truckdriver for a lumber company(lumber products)so i can get marine grade wood for considerably cheaper. any help on the subject would be great. i just want somethin fast, stable, safe, fun and atleast over 12 feet long. thank youo on the

oh and if anyone could find any info on the aqua racer and aqua rod for me to look at i would be very thankful. i think i'll switch over and build one of those.

08-14-2006, 10:47 AM
well i think i found a 2 cylinder 40 horse for free so im gonna build the x-1 and try thr 70 on it but if its to heay ill just throw the 40 on. and then if i need to ill build a tunnel hull. is there any way to make the back of the x-1 more bouyant if need be?

08-14-2006, 03:44 PM
Wider at the back, higher all across the back ... just like a tunnel hull

08-15-2006, 03:56 PM
would it be safe to modify the plans to widen the rear?

08-17-2006, 06:34 AM
ok on the plans the boat is only 17 inches high in the back so when i enlarge it top to bottom would i enlarge just the back or would i increase the height for all 5 of the frames? and how high do you think it would need to be to float my 70 horse?

Winger Ed.
08-25-2006, 11:36 PM
Whew,,,, I finally made it!

OK, let me address a couple things I've learned from the two hydros I built.


Turn fin:
Keep it. Maybe even put air traps running off it too. The air traps will help trap more air under the boat, lifting it a bit more out of the water at speed, and help stabilize it in turns. A hydro doesn't turn like a regular boat hull does. A boat turns like a motorcycle on dirt-- it sort of slides around the turn as you change directions, and leans downward in the direction of the turn. A hydro wants to lean out towards the outboard edge of the turn- like a car. When it does, you have to help hold it on the water and not let it 'slide' or the allow the outboard edge of the sponson dip down enough to go under the water... if you 'dip' a sponson, it can very easily, or more nearly, will probably-- flip over. And it will happen in about a millionth of a second too. Even if you're not building a racing boat- it ain't a toy either. These things are like pilots say about the old, and slow Piper Cub airplanes, 'They just barely go fast enough to kill you'.


For my first hydro, I built the 'Ben-Hur' from plans I got from the Clark Craft company. They are on the net, search for 'Clark Craft' or it will turn up while looking for 'boat plans' too. I switched it over from a wooden boat to all Aluminum, and built it to plan and raised the cockpit sides to handle me and the heavier engine ( 4 cylinder Merc 50 horse). It was OK. But it was still a 50 horse on a 12 footer......

For Poco Loco, I got the plans off the free 'boat plans' site. You have to enlarge them yourself, but they are free and they also have the plans for the hydro you posted with your engine picture. The 2nd hydro is Alum. too. I took the plans for the 'Airmarine Special' hydro- a 12 ft. 'D', and enlarged them.

I raised the top of the airfoils a inch, widened it 12 inches in the tunnel, moved all the 'ribs' to 14 inches apart, and added one more. That made it 16' long instead of 12', added a real chair & instruments, and raised the cockpit sides several inches to handle it setting deeper in the water with a stock 150 hp Merc. V-6.


Changing the dimensions on a boat can be done, just be careful to keep it all sturdy and safe. You will probably change the center of graivty in the process, but that is workable if you can move things around to compensate- like the fuel tanks or drivers station.

Something I sort of wanted to do, but didn't, was to make my 16 footer with 'wet sponsons'. The sponsons have a huge hole at the bottom of thier back ends to let the water out as you take off, but at rest, the boat sits more level in the water as they 'flood down'. If you have a center of gravity that is real far back---some say wet sponsons help it plane out faster by being front heavy on start off, then losing the weight as you come up & get going. Once you get moving, they drain in a few seconds, and forward motion keeps the water out the same way the water pulls away from the transom at speed in a normal boat.

Some sites you might want to check out:
I don't have the url, but they are easy to find on a search-

"Clark Craft". They have plans and the hardware you'll need like steering kits, turn fins, various epoxys & such.

"US Composites", they have all manner of epoxys and a decent price on the 2 pound per sq. inch foam you'll want.

"Boat Plans" They have dozens of free plan sheets on all manner of old boats.

'Antique Hydroplanes' Its a site where folks post pictures of old 1930's & 40's era hydros being restored, famous racing hydros, the evolution of hydros from the 'Rum Runnings' days to the present-- all sorts of stuff like that.

Good luck with your project, and keep us posted.


08-27-2006, 12:42 AM
whats the speed on your hydros? should i expect more or less of what your runnin? also on the turn fins how would i go about that just one in the center or what? this is new to me so im kinda lost on a lot of things here. my dad kinda helps but this hydro is a lot more to the 8 foot cab-over he built way back. and what modifications will i have to make. if need be i can show you all the plans and any info you need to know. again any help is helpful and i thank all who have helped thus far.

Winger Ed.
08-27-2006, 11:18 AM
whats the speed on your hydros?

The speeds I'm getting are much, much lower than similar boats built for competition. Plus, I weigh about 100 pounds more than the average driver who expects to have a chance at winning one of these races also.

The yellow 'D' with the stock 50hp Merc on it as well as the 16'er are totally foamed solid. That darn near doubles the weight of the hulls compared to ones built to race.

With me in the 'D', at 260 pounds, it does about 50 mph or a little better with a 16' pitch prop at 4,500-odd rpm. A similar, lighter weight hull with a smaller driver will probably do closer to 80 as the engine could wind up higher.

The white one with the (almost) stock 150, a 26" Chopper prop., and me in it does almost 90 at about 65-6,700 rpm or so. The foam in it adds about 200 pounds to the weight of the hull, and I carry a 16 gallon fuel cell. With someone weighing about 150 pounds, carrying less gas, it should easily get up to 100mph on flat water.

Your turn fin should normally be made from 1/8th inch Alum. plate.
I bring mine down off the inside edge of the rear ends of the sponsons. If you put one under the centerline, it will have to attach like a "T" along the keel. The fin/air traps you see on the yellow D are the same on both sides. They help trap more air under the hull and give it a little more lift to come up out of the water a little higher than just a turn fin alone**.

Keep in mind, these are 'ground effect' vehicles (that is the big surge upwards you feel just as a airplane is a few feet off the runway on landing.. You're coming down, then right over the ground you feel a big push up as air is compressed between the ground and the bottom of the wings). The top of the hull is an airfoil- like a airplane wing- that generates lift, as well the tunnel under it compresses air underneath- pushing the hull up. Since air resistance is easier to go through than water resistance,,,, The idea is to get as much of the hull up & out of the water as possible.