porpoising problem

[JK3]

I built a custom Bateau OB17. It's a c console bay boat with a modified vee and has a 70hp yamaha 2 stroke. When I get up to around 4500 rpm - wot she porpoises a lot. The recommended wot range is 5-6000 for the motor, with a 13-1/4" x 17" cupped aluminum prop I can only hit 5000 rpm. The motor has the a/v plate mounted even with the bottom, with 2 holes to move up. I've been in touch with the designer and he say's to move my weights forward but I already have a 12 gal gas tank under the casting deck and an anchor, chain and rode and some other equip. stored forward. She runs better with one person on the console seat. The problem is that I can't trim the motor up much at all and she starts to porpoise. I know trimming up would increase my rpms signifigantly but I really can't without the porpoising. <br /><br />I checked my hull and I do have about a 1/16-1/8 hook on either side of the keel over the a run of 4 feet from transom foreward. The designers say that rocker will cause the problem and not a hook. I've also read some conflicting posts here debating hook and rocker causing porpoising. <br /><br />A few guys said that I had the right prop and should turn the correct rpms when I get her not to porpoise and trim out. Should I go with a 15 pitch prop? Raise the motor? I will try to fix the hook with some epoxy and fillers but that's going to wait till winter.<br /><br />Sorry for the long post, any help is greatly appreciated!

 

[Gary H NC]

Re: porpoising problem

Do some reading here on Iboats about smart tabs..<br />Solved my porpoising problem..drives like a completely new boat.

 

[nevd]

Re: porpoising problem

Before I spent money on anything, I would lift the motor as high as you can go with your Al prop. A prop level with boat bottom is too low. If this does not solve your problems, borrow a stainless prop (probably 15 pitch) which you should be able to run higher than the al prop. You may need to check water pressure when the stainless prop is used. <br /><br />Higher motor mounting will increase transom lift (or less bow rise) and that is what you need to decrease the porpoising. The hooks you describe will also help decrease the porpoising so don't remove them.

 

[JK3]

Re: porpoising problem

Thanks for the suggestions guys. I agree about waiting on the tabs, that will be the last fix if all else fails. From some other boaters I talked to they thought a small 17 footer shouldn't need trim tabs. I'll do a little more testing and get back to you. Do you think I'm hurting the motor any by only turning around 5000 rpms (recommended 5-6000) and thats with my normal load of 2 adults, full tank and gear? I'm worried about when I'll have 3 adults on board and might be lugging the motor.<br /><br />One more - I know raising the motor will increase speed but will it increase rpms as well? Would it be signifigant or just 100 rpms or so?<br /><br />Thanks!

 

[steelespike]

Re: porpoising problem

Raising the motor will increase speed and rpm.The amount depends on the actual results.<br /> On some boats an inch higher can result in a 2 to 3 mph increase in speed.<br />Have you tried trimming through the porpoising<br />problem?<br /> Running at rpms below recommended at full throttle is not good.My info indicates a range of 4,500 to 5,500 rpm.It is possible if you reduce the weight in the bow it may stop the problem.Porpoising generally speaking is caused because the motor can't keep the bow up.Sometimes because of weight.Sometimes hull design.Weight concentrated at the ends of a hull can create problems sometimes.You might try emptying the front tank and try a 6 gallon tank in various positions through the length of the boat.<br />If all else fails smart tabs are an excellent choice and have expierenced excellent performace for lots of problem boats on this forum.<br /> They also improve performace caracteristics on boats without vices.

 

[QC]

Re: porpoising problem

Originally posted by JK3:<br /> From some other boaters I talked to they thought a small 17 footer shouldn't need trim tabs.

IMHO all v-hulls can benefit from trim tabs. I would want the boat to perform a little better without them too, but I would still get tabs. The low speed stability alone is worth it. Porpoise fix if necessary is a great bonus.<br /><br />I understand steelespike's point about getting the bow up, but porpoising is usually about too much trim or too little weight in the bow. I would not move weight aft. When the bow is too heavy your prop will blowout with only slight positive trim. If it propoises before it blows out, then more weight shifted to the bow is usually the answer.

 

[JK3]

Re: porpoising problem

I'm getting a little confused here about if I need more or less weight in the bow. No offense to anbody but I read a coulpe of other posts (not in this thread) that are conflicting. They can't seem to agree on what causes porpoising. From my expeirience with this boat (only used 7 times and still getting it dialed in) is that when I get into the higher rpms I have to trim almost all the way down to keep from porpoising. I can't trim out at all at full throttle and have to keep trimmed as far in as i can go to keep the bow from bouncing but I haven't tried to completely trim up and try to power through it yet. <br /><br />This weekend I'll try some test runs with different weights and trim settings. Should I still move the motor up a bolt hole?