Porpoising Help!!

[jamieat]

I have a question for you. I have an xpress 90HP Yamaha, 2004 model. I have a 6" jack plate and am currently running a 13.25 x 19 pitch Hustler vented 3 blade prop. I can get about 5800 RPM max with two people and gear. I have always had problems with the boat porpoising when there are two people in the boat. The jack plate and the prop helped a good bit, but the boat still porpoises when it is trimed up. Strange thing though, is that if I am turning just a slight bit to the left, I can trim the boat to the max, and hit max speed and rpm with zero porpoising. As soon as I straighten the boat up, it will porpoise terribly. With just one person in the boat, I can trim it up to max and go left, right, or straight with zero porpoising. This has become a real pain in the .... for me b/c I typically fish with a partner. Please help, any advice would be greatly appreciated.

 

[Jack Daniels]

Re: Porpoising Help!!

Have you thought about smart tabs, there are alot of good threads here regarding them do a search and you will be overwhelmed with info. I on the other hand just went with a hydrofoil, people here hate them, but im not putting smart tabes on a 20 plus year old boat. Dont think the money spent is worth it. Btw good luck, im sure others will have plenty of ideas for you.

 

[jamieat]

Re: Porpoising Help!!

Thanks for the response. I have done an ample amound of research reguarding trim tabs and I am considering them as an option. What I really dont understand is why my boat will run perfectly at WOT and at max trim with one person in it with zero porpoising. It will also run perfectly at WOT and at max trim with two people in it....as long as you are turning to the left. Just a slight turn to the left enables me to trim the motor up and run WOT. However, when I straighten the wheel, the boat will porpoise terribly. What is the reason for this?

 

[frozenokie]

Re: Porpoising Help!!

You might check to see if the trim tab on the cavitation plate is turned too far to the right. It's a quick adjustment, just be sure of where the trim tab is set before you change it in case it isn't the problem.

Good luck!

 

[LubeDude]

Re: Porpoising Help!!

All boats have a sweet spot as far as trim goes. Just because you can trim all the way up with only you in the boat doesn't mean that your boat should opperate the same when you change the weight! That's why the trim is adjustable. Just trim down a little when you have the extra people on board! There isn't likely anything wrong with your setup!

My Stratos Bass Boat is faster by a couple MPH when I bump the trim down just a bit from all the way up. Rooster tail is lower and it doesn't look as cool but it is more efficient. Rooster should as a good rule of thumb be about even with the top of your engine cowling at WOT!

 

[QC]

Re: Porpoising Help!!

The primary cause of porpoising is weight balance fore - aft. More weight up front will solve most issues, but then she'll be slower as you can't trim out as far. When you add that second body, you have changed the dynamics and with a prop that spins one way, seemingly weird things can happen. Trim tabs allow you to keep a lighter bow and still control porpoising. Another thing you might try is shifting anything heavy you can to the bow and visa versa . . . tools for life jackets as an example.

 

[jamieat]

Re: Porpoising Help!!

Thanks for all of the responses! As one of you said, the boat will not handle the same with two people and to trim it down. Well, I have to go from 5800 RPM's to about 4700 in order to get the porpoising to stop. However, if I am turning slightly to the left, I can achieve 5800 RPM at about 53 MPH even with two people. It is when I straighten the wheel that the boat will begin to porpoise. Very strange. Most boats go faster in a straight line and you have to trim down in corners. Mine is faster through the corners and allows you to trim higher!! Any advice?

 

[mtduckhunter]

Re: Porpoising Help!!

Rooster should as a good rule of thumb be about even with the top of your engine cowling at WOT!

my boat doesnt have a rooster tail at all. why could this be?

my boat porpoised too. i built a fishing deck in the front and now it doesnt.
the motor has 5 trim settings. it was in the 3 hole up and it porpoised, went down a hole and it didnt, added the deck, tried the 3rd hole up again and all was good.

hope you get everything lined out

 

[steelespike]

Re: Porpoising Help!!

Thanks for all of the responses! As one of you said, the boat will not handle the same with two people and to trim it down. Well, I have to go from 5800 RPM's to about 4700 in order to get the porpoising to stop. However, if I am turning slightly to the left, I can achieve 5800 RPM at about 53 MPH even with two people. It is when I straighten the wheel that the boat will begin to porpoise. Very strange. Most boats go faster in a straight line and you have to trim down in corners. Mine is faster through the corners and allows you to trim higher!! Any advice?

Have you tried going to full speed starting trimmed in and carefully trimming out to to a point just before problems.I have also been in boats with tiller steering that porpoised when I was sitting normally and stopped when I slid forward while remaining in the same seat.A slight weight change in your boat may help as well.Especially if you remove weight from the stern and add it to the bow or even midway.

 

[nlain]

Re: Porpoising Help!!

When you are in a turn there is a little more boat in the water, if you could hang over the side and watch you would see it, watch other boats running straight and see what happens when they turn. When you turn the boat lays over on that side and has a longer contact surface with the water, no porpoise, straighten out and you go back to just stern only in water and a long hull trying to pull the boat down while the engine is trying to lift it. If there is slight swells in the water that will make it start to porpoise, touch the trim down and it should go away, if the water is really rough you just will not notice porpoise because you are bouncing all over the place already. Yes weight balance is the key to porpoise or not.

I also highly recommend Smart Tabs, I have a set on my boat and they are well worth they price. The improvement in performance is great and the fuel savings will pay for them in a short time at todays prices if you use the boat a lot.

 

[jamieat]

Re: Porpoising Help!!

Well, I still have yet to get an idea that I havent already tried!! Other than the smart tabs(was trying to avoid purchasing them). I have also tried to redistribute the weight. Alone, the boat rides like a dream, like it floats on air. With two people, no matter how heavy or light, it porpoises like crazy unless you are turning left. My fishing partner is 6'2" and 265 lbs! I can have him move to the front of the boat, sit on either side, move from front to back, sit in the floor, on the deck, in the seat, it does not matter! The boat behaves the same no matter! I have a 6" jack plate with a 13.25 x 19 Hustler Turning Point vented prop. I also have a Power Tech 13.25 x 17 SS Prop that gives me less lift, less top end, and MORE Porpoising! I thought about going up to a 21 or 22 pitch to see if it would "carry" the boat better. I have also experimented with various adjustments with the jack plate and trim levels. When I installed the jack plate, it reduced the amount of porpoising by approx. 50% and increased my speed and rpm's significantly. I hope that there is a solution to this, it drives me crazy, I am losing 1000 RPM's and approx 8 MPH to this problem (unless I am turning LEFT or am riding alone). Anyone know of some cheap but good trim tabs that would help? How about a site to check them out? Brand? Style? Installation? Thanks! Everyone on here is such a help!

 

[nlain]

Re: Porpoising Help!!

Go here, this is the Smart Tab website. They have some good reading material on boat balance as well as info on Smart Tabs.

http://www.nauticusinc.com/

On the left side go down to Customer Service, the Balance & performance is the last Item, the other things are info on Smart Tabs.

 

[jamieat]

Re: Porpoising Help!!

Would you guys recommend me try a "fin" on the cavitation plate, or should I go straight to the trim tabs? I really hate drilling holes in my boat or motor, but I am willing to do about anything to resolve this issue. I have emailed Xpress boats, and amazingly I have yet to recieve a response (sarcastic response!). I am wondering if the hull of the boat is not blueprinted or something to that effect. Anyway, I am going to go up to a 21 pitch from the 19 that I am currently running as I can get about 5800 RPM' s out of it and see what change this has. If that fails, I guess I am going to have to search for other options.

 

[QC]

Re: Porpoising Help!!

No way on the Hydrofoil . . . IMHO, since this problem develops with 2 people but she is right with 1, then I would want adjustable tabs. But that's what I always say, so don't listen to me . . .

 

[mikesea]

Re: Porpoising Help!!

Is your jackplate adjustable,?if so you might want to go down an inch with the motor, or lower 1 hole.I have had the problem on a flatsboat and my 23 ft centerconsole that I just installed a bracket on.I went down 1 hole both boats stopped porposing

 

[LubeDude]

Re: Porpoising Help!!

What lenth and model of express boat do you have?

 

[jamieat]

Re: Porpoising Help!!

It is a 2004 H51 17' with a 2004 90 HP Yamaha. My jackplate is adjustable and I have experimented with various adjustments from the height at which it begins to vent and all the way down to where I can no longer achieve the RPM's I am looking for. Dont know what else to try!!

 

[jamieat]

Re: Porpoising Help!!

Anyone have any suggestions about trim tabs? Brands, size? Hydrolic, Pneumatic, manual, fixed? Which ones would you recommend for my setup and boat size? Thanks

 

[nlain]

Re: Porpoising Help!!

For your boat I would go with Smart Tabs.

http://www.nauticusinc.com/

Iboats sells them here. I have a set on my boat and they will do everything advertised. They are well worth it. Great product with great support. If you use them you will get on plane faster, low speed wander will go away, stay on plane at slower speeds, save fuel, handle rough water better, porpoising should go away and the boat will generally ride and handle better.

 

[QC]

Re: Porpoising Help!!

I don't agree ^^^^^ because it is right with 1 peraon. I would go Bennett's adjustable. Allows you to use as little or as much tab as you want for the moment. Because there is something directional going on as well, I would want to be able to individually adjust port and starboard tab.