• If you log in, the ads disappear in the forum and gallery. If you need help logging in or getting registered, send request to: webmaster@southernairboat.com

Big block or small block

graffixx

Member
I have been having issues with my 13 ft predator it seems to really want to ride on its nose. The motor is dead level. When I accelerate hard it runs both but as soon as I start to let off it porpoise terrible. Long story short I blew the small block up yesterday and I am debating weather to put another small block or I have a line on a 454. I am thinking the extra wieght on the back may pull it off the nose and if course the extra power is good also. I put a new coat of slick and left it a little thicker in the back and it was almost unsafe to ride so I ground it down to make it a little better I may just need to ride it on the asphault to solve that part but do I go big block or small block
 
First be aware, that in general with a V8 and a belt drive on a 13' hull, you are coming out of the gate at least a little ass heavy.
Not to imply that isn't something than can't be done successfully, but rather is something that can not be overlooked.

Naturally, when the bottom is straight and true (and strong enough to not flex), and there is no intrinsic weight imbalance, the running attitude of the hull can be adjusted comparatively easily by trimming the angle of thrust column by shimming the engine mounts.
Realistically, either or both scenario(s) can be expected to complicate the effort required to attain the desired end.

Your description of the engine being level (with the stringers) and wanting to porpoise when you let off the gas implies one, if not a combination of both possibilities.
The first being, that the bottom is not straight, true and/or stronger enough to not flex. In this scenario, installing a jack to stiffen the flexing stringers and/or to restore the planing surface to straight and true would be the way logical way to proceed.
The second being there is potentially too much weight forward. While this seems unlikely, is the tank mounted forward or is the seating configured in such a way to offset the stern being heavy? If so, it's possible that while that effort could be successful, it could be also be contributing to the hull wanting to flex rather than stay straight and rigid. (pictures can be worth a thousand words)

In either case, your success with minimizing the issue by grinding down the rear of the hull would also imply that the bottom is not straight.
If there were hypothetically a hook (dip) in the hull, it would be minimized by grinding down the aft planing surface at the stern.
Your experience seems to have already shown this to be the case.
The same thing can be accomplished by filling in the low area forward; BUT ONLY WHEN the hull remains straight and rigid without flexing.

If you were to run some string lines (my preference) or use a five foot level on the bottom, you should be able to confirm or deny if there are low spots (especially forward of the rear planing area) or there are not. If you find that there are, that would be your preferential jack placement. If you find the bottom is dead nuts straight, then it would be safe to assume the bottom is flexing and needs stiffened up.

As far as your engine replacement goes, I do not believe simply going back with a heavier engine has any realistic chance of magically helping you trim the hull to run like you'd like. It possibly could I suppose, but I would hold no such expectation.

Based on the way you describe things, I think your hull needs some help to attain the end you are after. If you determine what that help is, and then provide it, I'd think you can attain the end you're after with either engine you opt to use for whatever reason(s) you make that determination. With that having been said, YOU have to make the determination if YOUR hull will accommodate a big block in addition to your belt drive, I'm simply not familiar with it.

And yes as you've noticed, it is difficult to escape the recurring theme here of having a straight, true and sufficiently rigid hull.

ps There is more detailed info about hull tuning here: https://southernairboat.com/phpBB3/viewtopic.php?f=85&t=28600&p=276852
 
I can only say wow that is the best explanation an answer to my question I could have ever asked for yes I did straight edge the bottom of the boat and it is very straight before I put this last coat of slick on when I put this clode of slick I pulled it to the back with about a quarter inch of filled on the last 5" To try to accommodate my problem my seat placement and such I feel is pretty good it was a predator built boot and has the original rigging I will add a picture also The bottom of the boat could be flexing but I never really thought about that because the predator 2x6 on the rigging stringers But certainly possible the boat topped terrible before I put the slick bottom on it and after I put the slick bottom on it with that build-up at the back it didn't hop but like I said it felt like it was trying to dive under the water and seem like it actually tried to a few times enough to poor water over the bow lol. My question about the motors Was because I was looking at A big block and a small block but neither one of them is going to stay in the boat very long because I am building a different motor but is not ready at this time and if the big block would work I would have a better motor for another project when I take it out. Thanks again you certainly answered the questions that I asked in perfect detail
 
Back
Top