Lugging motor? Prop choice and porpoising

[JK3]

All right guys, I'm worried about hurting my outboard. The boat is a custom built OB17 center console v-hull designed by Bateau. It has a 70 hp Yamaha 2 stroke (2005). The boat wieghs approx 1500 lbs with 2 adults on board, full tank and gear. I'm running a Turning Point aluminum prop 13.25x17 and hit 5100 rpm at WOT at 35 mph. I do have a problem with porpoising (around 4500 rpm) unless I keep the motor trimmed all the down at wot. Normal cruising on plane is about 22-25 mph at 3500-4000 rpm. Am I lugging the motor at those loads and rpm? I believe I would get a few more rpm's if I could trim the motor out. The AV plate is even with the bottom of the hull and on the 2nd hole. Would moving up a hole make a difference and should I change props?
Any help would be greatly appreciated.

 

[LubeDude]

Re: Lugging motor? Prop choice and porpoising

Yes you are lugging the engine at those RPM`s. IMHO you are under powered for that boat and you are not likely going to be able to trim it much at all without porposing reguardless of the setup. Probably not what you wanted to hear.

 

[walleyehed]

Re: Lugging motor? Prop choice and porpoising

First item I would consider is weight distribution...sounds like you got too much weight in the aft portion of the boat. 5700-5800 would be a good goal to shoot for with a normal load.
You say "custom-built"...how old is this rig and have you checked for a hook in the hull right where it meets the transom?

 

[evin268fs]

Re: Lugging motor? Prop choice and porpoising

have been browsing the forum a bit and have seen people refer to a hook in the hull, what is this? Just curious.

 

[JK3]

Re: Lugging motor? Prop choice and porpoising

I built the boat myself, stitch and glue construction and splashed it for the first time in April of this year. The hull itself weighs 750lbs without the motor and the plans recommend a 70 hp. Don't think total weight is an issue. She jumps up on plane very quickly. There have been many other boats built that use a 70, 60 and even 50 hp. I think you might be confused with the production Bateau boats.

I do have a little hook in the hull about 1/8" to maybe a 1/4" deep a two feet or so forward of the transom on both sides of the keel. As far as wieght distribution there is a 40lb battery in the port side transom locker and that's it in the stern. Up front there is a 12 gallon gas tank under the casting deck and an anchor stored up in the bow. I think I have my weight distributed pretty evenly.

 

[Dhadley]

Re: Lugging motor? Prop choice and porpoising

A 1/4" of hook is a lot. Any hook should be right at the back, not 2' forward. That's not going to help the porposing issue at all.

 

[LubeDude]

Re: Lugging motor? Prop choice and porpoising

With the hook and the weight of the fuel in the front and the 70 HP, you are not going to be able to use the trim much at all.

Congratulations on building the boat all yourself, that`s quite a feat. Pictures?????

 

[JK3]

Re: Lugging motor? Prop choice and porpoising

Sorry guys I didn't have access to a computer for a couple of days. Here's a link to some pics:
http://209.190.4.227/gallery/displayimage.php?pos=-8030 Anyway, any suggestions, lower pit...p speed, I just don't want to hurt the motor.

 

[LubeDude]

Re: Lugging motor? Prop choice and porpoising

That`s a great looking boat and it looks like you did a great job. Try a 15P prop and see if your RPM`s come up. A stainless cupped prop will allow you to get a bit more bow lift but I still think with the 70HP and the Hook and the weight of the fuel in the nose that you may be fighting a loosing battle.

 

[JK3]

Re: Lugging motor? Prop choice and porpoising

Thanks for the comments LD. Did a little thinking over the weekend. Do you mean I'm creating a pendulum action when the bow is lifted with the up trim, then at a higher speed the hook creats lift in the aft section of the boat forcing the nose down, which slows the boat down, and the process starts over again?
I'm going to fix that hook with some epoxy and filler. The problem happens time to time with some of the other builders and they have have fixed the problem that way. The designer suggests a max of a 70 on that boat due to the light weight and recommends the gas tank up front.
I do have up trim at slower speeds, when I get to around 30 she begins to porpoise so I have to trim down to control it.
If I fix that hook and get a nice flat planing suface I should be able to fully up trim the motor at wot and free up a lot more rpm's.

 

[Dhadley]

Re: Lugging motor? Prop choice and porpoising

A completely flat botom will hurt the holeshot. And possibly raise other handling problems. You'll want about 1/8" to no more than 1/4" hook right at the rear. You'll want less at the "V" and the most at the chine.

How many lifting strakes are there? 1 on each side or 2?

 

[JK3]

Re: Lugging motor? Prop choice and porpoising

I'm not too worried about the holeshot. There is about 6 degrees of deadrise at the transom so it's pretty flat back there. I think that should help getting up on plane. There is a skeg aprox 1" wide x 1.25" tall and a lifting strake on either side which really helps with tracking. I've thrown it into some pretty good turns and it hasn't side-slipped at all. The hook is actually a 1-2 feet forward of the transom, I had a 2x4 support under the hull in that spot. I think that area deflected a little bit when I climbed into the hull to glass it before the stringers were locked in.

 

[LubeDude]

Re: Lugging motor? Prop choice and porpoising

It takes a lot oh HP to hold the bow of your boat at full trim. I have had two 15' Bass boats , one with an 85, and one with a 75, neither would allow me to trim max out. I had to just bump the trim untill it bounced a bit and then bump it down. My present 17' Bass boat with a 150 is the first boat I have had that would hold the bow up at full trim but it`s a bit faster if I trim it down just a tad. Rooster tails are fun, but not efficient.