Too much HP

[Johnny M]

Is there such a thing as having too much HP? I am looking at replacing my engine that served well  (1680 hrs)  with a corvette engine that is built up a little and is running around 385 Hp I have a 1994 AM TBX and would like to know if there any concerns with engine mounts, transmission, etc.

[Mike Harry]

Look up Backfoot100 on this forum. He built a 383 for his Barefoot and I believe it was called a "fire breathing monster" by Ron Tanis after the second American Skier reunion. Eddie did a great write up about the build.

[RonT]

Short answer yes. I wrote a lengthy post explaining this then was booted off 
Need 1.125" shaft for 300HP+
Need all welded alum. engine bed. (some of Wesmars were not)
400-450HP will max out the design, 375+ makes you the test pilot

[Midskier]

lets see how much brothers agree.........

Short answer - NO lol

yes need 1 1/8" shaft for 300+

yes need welded engine bed

the 3 event tournament style ski boats are not not your typical hull. Hulls which are generally categorized as either planing or displacement:
a bass boat would be an example of a planing hull it waddles around up to 15-20 mph then it's " on plane" add more power it goes faster
a sailboat is an example of a displacement hull it will go faster with more power too (wind) but uses the hulls displacement to help stabilize it

ski boats are somewhere in the middle - they don't have a planing speed they can hold any speed in 1 mph increments something a typical planing hull can not do
there are  hull tricks and tweaks in ski boats to add lift @ certain speeds and reduce lift @ others, the TBX is pretty complex which is why it's such a great skier

having a boat with 260ish hp and seeing 42-44 mph is fairly common, adding 100 hp may or may not improve the top end a whole lot maybe 2-3 mph add another 100hp may only net another 1-2 mph
somewhere along the line with speed the hull starts to push or displace more water  so not only are you pushing the boat but the boat is pushing more water - like a snow plow, hence the small increases in speed

I have customer with an '18 skier with dynoed 480hp small block - she runs 44mph - dropped $5k for 2 mph

Are there 50+ mph ski boats - sure there are , and several guys have installed thunderous motors with tons of hp, something the mfg would never have done for the general public because some hulls may become unstable above certain speeds.
Sure it may be a fun ride now and then but the operator needs to be fully alert - like a 16 CC mustang with 350hp - it might do
55-58 mph but it certainly ain't safe

so there you go - plan on using your 385 hp corvette motor - add the correct shaft size (and strut, coupler, prop) inspect and beef up the engine bed and enjoy - will it work - probably, will it do 60 mph - no, but when tuned correctly it will be faster than the original engine and most likely quicker out of the hole as well and above all more fun to drive knowing what's under the hood

so is there such thing as too much hp - in general NO - in our boats YES

 


** tuning hee hee hee - where and when an engine makes its power and the load placed on it has been pretty standard in the direct drive ski boats deviating in big steps usually ends up with poorer than original results.


good luck

Dan T

too much HP ?

I've had lots of high HP toys modded for more - yes there's certainly a thrill, a rush - and yet a compromise

my 120ci 120hp v twin - certainly wasn't impressive
though the 150 hp crotch rocket was very impressive
a 70 hp snowmobile is a ton of fun - the same chassis with 160 hp is a completely different dimension
jack it up to 210 hp and the thrill is multiplied - along with the risk of death :0
(which by the way all toys mentioned above are now owned by some other thrill seeker ) 

[Johnny M]

Thanks to you all,, the reason I am considering this engine is simply economics it has the exterior modifications I felt I wanted, the heads etc. are overdoing it a bit but for $1200 it makes sense, my present motor has 1680 hrs the boat is in incredible shape , gel coat still looks new, all aluminum and fiberglass bed 1 piece of wood for the observer kick plate .  I love the sound of a well tuned mild cammed V8 and I like lifting the hood much like Backfoot does, its mostly ego and pride we are talking about here. But if I can get close to 50mph I would be more than satisfied I do agree that alot of tuning other than engine will change hole shot and top speed. so I will be asking the brain trust of this fantastic forum for help

[RonT]

I give up, just typed a shorter response (1/2 page & got kicked out again)
300-350 HP is fine, should get you near 50MPH.

[RonT]

Here is another short answer from one of my books that explains it as simple as it gets: "in rough terms, a boat is planing when it's travelling faster than its transverse stern waves, which occurs roughly at a speed in knots of 3.4 times the square root of its transverse beam in feet. Full planing is about twice that speed..."
In other words, all American Skiers are planing hulls, in fact the Queen Mary could be a planing hull if enough thrust is applied, or you could say the Unlimited Hydroplane Miss Budweiser could be a displacement hull if you remove the designed thrust with a 10# trolling motor. The hull as designed by the designer includes the thrust parameters which will determine if the boat is a planing or displacement hull.

[Joel]

Ron,  As usual, you lost me at the bakery...       But I get your point.

Guys, if your looking at a bigger motor then top end shouldnt be the objective... for all the reasons stated above... the hull wont allow it or its just not safe at much over 45 mph...  you'll be bouncing like a jack rabbit if you hit some wake.  A  stronger / faster hole shot should be what your shooting for...  They call these tow boats for a reason... they have tow boat style props for low end grunt and flat bottoms to help smooth out the wake...

Put in that big engine, reinforce the pylon stand / engine cradle, put on a new shaft & prop and pull up a few slalom skiers at the same time...  Now THATS braggin' rights!!!   

Joel

[RonT]

I remember building a few show ski boats back in the day, Advance hulls with 375 HP 454inch motors, massive aluminium engine bed with some serious pylon support. All they wanted to do as I recall was to pull 10 barefooters at 40mph.

[backfoot100]

I concur with at these guys are saying but there are a few caveats that will affect the end result.

First off, I'm assuming a 94 TBX would probably weigh a little more than my 86 Advance. I would guess at least a couple hundred pounds. Maybe even more. Might not sound like much but on same hulls with like motors it would be noticeable.
I also have to assume the hull design itself is different then the Advance. I would have no idea how that translates to hitting your desired results. Ron's explanation was WAY over my head. I've never personally had the chance to examine the differences between an Advance and TBX hull.
I would also caution what cam that engine has in it. Probably works great in a lightweight Vette but in a boat, I'm guessing that same cam would probably suck. I would recommend doing some research and look at changing out that bump stick for one more suited for marine use. I know of guys that used an automotive or truck grind cam in their rebuilds and were very disappointed.

I would also look closely at the freeze plugs in that engine. I have no idea if an aluminum block and heads use steel or some other material. I suggest refitting with brass plugs if they aren't brass already.
I also recommend changing out to marine specific parts that engine will not have. Carb, distributor, starter, fuel pump, circulation pump and alternator. I would hate to see that bad ass motor go up in flames (and more importantly, you and your guests).

Either way, 50mph should be attainable but how fast you get there will be amazing.

Keep us updated and Good Luck.

[Johnny M]

Thanks Backfoot ,  after the advice and knowledge from this forum and the engine I was planning to use had a very wild cam which did concern me a bit  (interesting you singled out the camming aspect) and the horse power being a little outrageous I have decided to but a new gm crate engine 350/290hp short block and add the balance of marine parts from my old one I think the TBX was 2250 lbs I will add a little bling of course and also new xmanis similar to yours I will search for them in my neck of the woods (Canada) but  I am sure will end up buying  them in yours.  Ron and Dan you are both remarkable people offering your insight and bountiful knowledge I will post pics etc. it helps newbies like myself immensely to read about others projects. Thanks