Need help with new boat porpoising problem

I just completed my 4ft by 8ft Minimost boat. It’s an 8ft long by 4 foot wide wood and epoxy glass boat that weights about 100 lbs. It’s a monoplane / hydroplane type boat with a relatively flat bottom. Its powered by a 18hp Tohatsu outboard with remote throttle.I spent about $3000 to build it and many many hours. I am afraid it may be firewood after all this time and money


Here is the problem. The condition of the water was perfectly glassy smooth. The boat porpoises uncontrollably once it reaches a plane. The front end just bounces out of the water uncontrollably no matter what the speed once on a plane. We tried shifting the driver’s weight forward and this made no difference at all. We even tried lying on the front deck as we were driving. We tried every possible angle and degree of adjustment in the motor trim. We also tried every different weight driver from 100 lbs. to 225 lbs. It does the same thing no matter what the adjustment. We also tried putting 75 lbs. of weight in the hull at the front of the boat thinking that there was too much weight in the rear. This also made no difference at all. What can we do to remedy this situation? My two thoughts were some type of a whale tale fin on the anti- cavitation plate of the motor or maybe trim fins on the boat. Can anyone offer a suggestion to solve this problem?

 

To start with, an 18hp Tohatsu is a HUGE engine for a boat that tiny. At what point on the throttle does porpoising begin? If you can plane at low throttle OK, and the problem's only as you get faster, you may have too powerful an engine for such a light and short hull. (My own boat is double that length and weight, and it went like stink with its old 15hp.) If you can borrow a shortshaft 8 or 9.9 for a day, try running that and see if it behaves better. Overpowering is a very common reason for difficult control.

Also check: Is the motor shaft length correct? If you put a longshaft engine on a shortshaft transom, the prop sits 5" too low and you'll get some very strange behaviours. Is the motor height correct? (antiventilation plate at, or very slightly below, the bottom of the hull). Too high or low will give propoising, skidding, falling off plane, etc.

Whale-tails, trim tabs, etc. help to get a boat up on plane, and on some boats improve stability. But I'd advise against going this route yet, as this gear will only be of benefit if the boat is already running OK; they improve performance but can't create it.

 

Hi Marshmat. This is a very popular boat (see minimost.com) Many people put much bigger motors on them. Some people have run 30hps but they dont recommend over 20hp.Most have run 20s very successfully. The boat only goes probably 25mph max with the 18 so I was hoping to go a little bigger. I thought i was underpowered.

As far as what speed does it start at . It starts as soon as it gets up on plane and never stops. It is violent at all speed. I cant ever run it slow.

As far as motor height, I built the boat to fit the motor. The anti cavitation plate is exactly level with the bottom of the boat.

I think the only solution may be trim tabs , but the boat doesnt have electric or hydraulics. I dont know if you can purchase mecanical ones.

Any help would be greatly appreciated. I have tried everything.

 

Hi again Marshmat. I forgot to ask . When you say antiventalation plate. Do you mean the bigger plate just above the prop or the smaller one above that? I have the big one above the prop dead even with the bottom of the boat.

 

That boat looks very familiar.... I've seen boats like that racing in A-Stock Runabout classes, I believe. (A-stock hulls are 8' by 4' and run a 15hp shortshaft outboard.) At least they look similar, it might have a different hull. A friend of mine at the cottage has a 12-footer with a 25hp that is a very similar design.

The plate I referred to is the big one above the prop. It appears you have the height set correctly. You are only getting 25mph because of the porpoising; you should be well into the 30s with that motor once you're running level.

It sounds like you have the motor trimmed in all the way. Try placing a broom handle along the keel and see if the plate is tipped up or down relative to it. The motor should probably be on the 1st (closest to hull) trim pin setting, in running position.

Do you have any photos of the boat, particularly the underside and transom? It would be helpful in figuring out why the boat is doing this, and would give a better picture of what kind of devices might be appropriate.

 

Hi Matt. The motor is in the first position, but we tried it in all positions. Most of the time we ran it with the cavitation plate parallel to the bottom of the hull. We tried it below and above that. We did run a straight edge off the bottom of the hull. We tried it tipped up and down. It did not stop the problem. The porpoising did get a little better as you went faster but never got to the point it was rideable.

I will borrow a camera and take pictures tonight. I will send them to you tomorrow.

Thanks again. Appreciate your time. . Scott

 

I overpowered allot of little boats when I was a kid.And I have used a thing called a doalfin or dol fin or some thing like that it bolts on to the cavitation plate like wings.It sucks on slow boats it makes them even slower.But on over powered ones it helps big time to stop porposing and they are cheap It is worth a shot I bet you don't even have to order them just go to the marine store and get them.you could also make a cool set of homemade trim tabs with a couple of stainless piano hinges,2 alluminum plates,and 2 turnbuckels to adjust them,sort of like on a jersy skiff.

 

Doel-Fin, a particular brand of whale-tail. There's about 50 different brands, all of which claim to be the best. I've seen home-built ones of scrap aluminum plate that work. From what you say about what you've done with the motor, it sounds like you still have more lift up front than in the stern, which the plate could help with. It does involve drilling holes in your motor casing; but since you've tried everything else it might help.

 

What is porposining???????????????

 

Porpoising: An undesirable running condition of a planing hull, where the bow rises and falls repeatedly when on plane.

 

Have you run the numbers on porpoising?

 

Hello all and thanks for the help and advice. I have a full set of photos on my computer. Is there a way to post them with my reply? I have no idea how to run the numbers on porpoising. I will try to get the photos posted on the www.minimost.com website. That is what type of boat it is. If anyone wants to give me their email address I will send them a full set. I just dont know how to load them here. If not I have already purchased the Sting ray Junior hydrofoil fin and I will try it based on your advice. It was $50 but I guess that was cheap compared to how much I have sunk into this boat. If that doesnt work I will make some mechanical trim tabs with turnbuckles and stainless steel plates and piano hinges. Thanks for the help . Scott

 

Ah I figured out how to attach photos. Here they are . I will have to post another reply for the other photos.Scott

 

heres the rest. Hope this helps the issue. Scott

 

Nice lookin' boat. Overpowered, huh? weight won't help. length and beam, yeah.