help with new boat setup

[chris3298]

I'm not sure where to post this at on this forum so here goes. I've got a 15x52 alweld Vee La aluminum boat which is really a V bottom all the way to the back of the boat not just a semi V in the front like some aluminum boats. I have a 2011 Tohatsu 50 TLDI shortshaft stock aluminum prop 11.1x14 I think. This boat was bought for fishing and well to play with and go fast. The boat has a 17 inch transom aluminum trim tabs welded to the boat that came standard with the boat, it has a deck at the bow and a bench seat in back.

I took the boat out for the first time Saturday with the motor completely down on the transom and motor trimmed fully down boat ran great at full throttle with my dad who weighed about 250 sitting on the deck on the bow. Absolutly no porpoising. When I went to trim the engine up not very much it started to porpoise. I thought you wanted to be able to trim the motor up some so I could maybe get more speed out of it.

I'd just like to get more speed out of the boat and would like to know what advice can yall give me?

I forgot to add I have a battery just behind the rear bench seat and the gas tank I have now mounted far forward. I did notice that one of the trim tabs was bent up while the other was level with the boat. I bet it was bent during shipping. I made sure they are now both level with the bottom of the boat. I also raised the engine up two bolt holes to try and get the cavitation plate level with the bottle of the boat and it is but of course I trimmed the motor up so that cavitation plate was level with the bottle of the boat in reality I will probably have to have the engine fully down tomorrow to run the boat and keep it from porpoising so in a sense the half the cavitation plate will be below the bottom of the boat because I will have to run the motor fully down.

So what can I do to achieve more speed?

Thanks
Chris

I have probably confuses the hell out of yall but want to give you as much information as i can to help me have a better setup.

[zul8tr]

Can you post some pics of the rear with trim tabs and engine. Need to see the size of the tabs.

Based on the porpoise operation you note I would trim the engine out so the prop shaft is level with the bottom and note the ride, Then from there adjust the trim tabs angled down to force the bow down. Unfortuanately you state the tabs are welded so a hammer is the tool. If that provides success then raise the engine on the transom and check the ride in the straights and the turns. Generally in turns an engine to high will cavatate.

Experiment is the only way and keep notes.

Tell us the results

[chris3298]

Thanks so I should probably drop the engine back down all the way on the transom and go from there adjusting the tabs.
I don't have a digital camera but will see if I can take a picture with my laptop this afternoon

Ok now gotta figure out how to post a picture

[chris3298]

The rear and sides of boat, sorry if the image isn't to great if it is hard to see I will try taking my boat out the garage for better lighting. I dropped the motor all the way down like you said and I used two pieces of 2x4 with 2 C clamps and bent the trim tabs more level with the bottom maybe even a little past level.

[zul8tr]

First place engine flat on transom with no shims then adjust the engine prop shaft level with the bottom and run it and get a base line reading of performance, Do you have a speedometer and tachomerter?

Those trim tabs look rather small and they might need to be increased and made adjustable without bending.But try them as they are by bending.

[chris3298]

Just back from the river, first thing i noticed is well i may have bent those trim tabs a little to much past the level part of the bottom and noticed more spray. Well the thing got on a plane quick and the nose was down was thinking dam maybe i bent them to much, ran WOT and was thinking dam my fat dad in the front it seemed faster then i forgot to trim the motor up, I did with just very small burst of the button ran like that a while till I got use to it. I then said ok time to trim and get this motor level and well it was tuff to tell I think I was pretty close to being level meaning triming the motor up. I wish I would have brought a tape measure to then measure the bottom of the tiller handle to the seat and make this measurement again when on dry land to see just how level the motor was and forgot the dam gps to but the motor was trimmed up alot more then the first time out. I felt like i was a lot faster triming it up, there was times that it was porpoising but was able to get on the throttle and work the porpoise out. Today was very windy and not a good day for fishing but barely good enough for testing the boat on the river.

Something I noticed was when trimmed up and wot it didn't feel like much of the boat was running in the water, at times the wind or current seem to catch the boat and want to move me over, it felt like, the back end was very loose meaning felt like not much was in the water but felt dam fast. Almost like it was easier to move the tiller but at the same time very easy to get out of control.

I do have a question about the tabs, a friend of mine here in Louisiana had a custom built boat much heavier but the rear of the boat looks like the way it was made they extended the bottom plate so he had about 2-3 inch lip as like a trim tab like mine but was across the entire back of the boat. There was a small cut out for where the lower unit was. I'm just curious to know why this was done and in my case would it help it any for those trim tabs to be longer?

Another guy with same setup as me on another forum has real porpoising issues but mine don't seem to be to bad I hope.

Forgot to add the motor was fully down on the transom this run. No tach but have seen a brand called Tiny tach which is a digital tach but not sure if it will work on this motor. Can you recommend a tach?

Thanks
Chris

[zul8tr]

Sounds like progress. Need to bring that gps next time. Have you been keeping notes? The tiny tack is OK just inquire with them if the model they offer willl work on your engine. They used to have a series of different models but now they offer one that has settings for several different engine types. I have the older version 1C that was for a specific 2 cycle engine that works on my 1973 Merc 25ss CD ignition 2 cylindere and the yamato Y80 2 cycle 2 cylindere standard point and condenser ignition.

As far as the bottom extension you note forget that and stick with the tabs and experiment with bending. If that gets you running right you are set, If the tabs were larger in both directions that would provide more surface area for trim most likely at less of a down angle to achieve that same effect as the smaller tabs. However if the tabs were adjustable they would be easier to note the position and a finer adjustment allowed.

The next phase is to start raising the engine on the transom and adjust motor angle and tabs to suit the new height.

Keep notes

Do you know the prop diameter and pitch and engine lower unit gear ratio?

[chris3298]

I have not kept notes but really this is truly the third time i been out in this boat. Second time i moved that motor to high and was moving to fast in the adjustments.

I was gonna run today but it is raining, will have to tomorrow. I took the advice of someone on another forum and got my boat as level as I could with a level and then tilted the engine up to where the level read level on the cavitation plate and I made two marks so when I am back out there and have trimmed my motor up as high as I can go to the point it doesn't porpoise i can then stop and see if the marks line up and if the motor is level.

Depending on if the marks line up I will either bend the tabs down more or like you say raise the motor up one hole. Speed is definitely a slow learning process I realize now.

Tohatsu website says the gear is a 1.85:1 the prop is an aluminum 11.1x14 this prop came with the motor. I noticed yesterday this boat takes off really well if I can tame the porpoising as I raise the engine I think I could maybe get a good bit of speed out of it, I hope we shall see.

I did put a straight edge under the boat now it isn't as perfectly flat as I would have liked but the boat seems to have a little bit of a hook. I understand it's tuff to get a boat flat this thin of aluminum and aluminum likes to warp like mad when you weld it.

Out of curiousity anyone here on this forum ever build there own aluminum boat? This is something I'd really like to do. A boat built for speed

Thanks
Chris

[zul8tr]

As a point of reference with a 14 pitch prop and a 1.85 gear ratio here are some theoretical speeds

at 5000 rpm about 36 mph
at 5500 rpm about 40
at 6000 rpm about 43
at 6500 rpm about 47

Calculated from MPH = 14 pitch in inches x RPM / (1056 x 1.85 gear ratio) = RPM / 139,5

These speeds assume no slip and that is impossible so expect somewhat slower mph by about 10% of those calculated..

The hook in the bottom is like some down angle on the trim tabs it will put the bow down somewhat, Unfortunatly a hook is not desirable.

Keep working on it

[chris3298]

Thanks for the information, I must say this bot is 10 times better then what i had. I had before was a J14 carolina skiff it weighed probably about 550 and had to two rocker and one really bad hook. That was the worst boat I ever had this boat is just about perfect compared to that one haha.

I guess really the only way if you want to go fast is to build your own. I've learned my lesson you want it done right you gotta do it yourself.