Outboard cav plate fins

[Bigman]

I recently bought a 20 foot Thunderjet Falcon with a 115 Yam. While on sea trials to break in the motor I noticed that even with adjusting the power trim I still got a bit of a "bounce" when it got up on plane in flat water. I can trim it to put the bow down to remove the bounce but then it's not planing properly and the boat speed drops a bit.

Years ago I had a 15 foot Harbercraft open boat with a 40hp Yam tiller steer with the cav plate/fin attachment. With me (a big oaf), the motor and the 30 litre fuel tank all in the back and nothing but a fly rod and a six pack of Lucky in the bow, the boat would plane at 1/3 throttle and the "bounce" was non-existent. Before I installed the fins the boat would do the very same bounce thing at cruise/planing speed. Not only did the fins help with planing and this bounce thing but I picked up a few kms/hour in top speed and the boat handled like it was on rails. I'm wondering if I could expect the same kind of improvements if I put the fins on my new, larger motor and boat.

Do any of you fellow alloy boat owners have the fin on your outboard and if so, did it make a positive differeence?

Thanks in advance for the sage advice and opinions,
Bigman.

 

[Captain PartyMarty]

Forget the Fin and use your trim tabs, if you don't have trim tabs install them

 

[ReelSlim]

I put an SE Sport on my 21 Striper and it definitely improved the ride and fuel bill a lot. I have tabs but only use them to balance the load a bit and it works great for my set up. I had them on my last boat which had heavy Honda power and was butt heavy. It made more of a difference on that boat but it was a bit smaller. With my current boat I notice the biggest improvement when coming back from offshore in a following sea. I find my boat will lurch a bit with the tabs down in a heavier sea. Every boat reponds different to "hyrdrofoils" but I have had good success with them. But as mentioned already tabs will do the same thing just pricier and if you boat rides level then the foil will stop the bounce.....IMO.....Cheers!

 

[trendsetter]

Two of my boats came with the stingray fin installed. One boat, a 16.5' stratocraft porpoised like crazy when I removed it so I put it back on. The other, a 17' Grew performed much better with the fin off.

They tend to work well on stern heavy, under powered boats in my experience. The best option to correct porpoising is weight distribution. Next best is hydraulic trim tabs, then Smart Tabs (maybe the best option for you), a stern lifting prop like the Solar HR Titan and engine mounting position. Any of these may help your situation.

If you already own the fin then I'd say try it otherwise go for smart tabs. Smart tabs are less expensive than hydraulic trim tabs and work well from what I hear.

 

[triplenickel]

The best option to correct porpoising is weight distribution

Yup, play with weight distribution. Get some sandbags to experiment with, it may be as simple as putting a battery up front.

 

[Island Fish Lifter]

forget the fin and use your trim tabs, if you don't have trim tabs install them

x2
.........

 

[Kildonan]

Get some sandbags to experiment with

I know two sandbags. One named Jim one named Pat. ;-)

 

[GLG]

I recently bought a 20 foot Thunderjet Falcon with a 115 Yam. While on sea trials to break in the motor I noticed that even with adjusting the power trim I still got a bit of a "bounce" when it got up on plane in flat water. I can trim it to put the bow down to remove the bounce but then it's not planing properly and the boat speed drops a bit.



Do any of you fellow alloy boat owners have the fin on your outboard and if so, did it make a positive differeence?

Thanks in advance for the sage advice and opinions,
Bigman.

See your dealer to tune your boat.
The dealer should be able to fix your problem.
After all you have paid him large so he want's you to be happy.
It could be as simple as changing your prop.
I went from the stock Yamaha 13X19 to a turning point hustler 13 1/4X19 and that fixed my similar problem.
This spring I will swap out that prop for one that is an inch or two smaller (13X17).
I lost bottom end power so I'm trying to get that back.

I'm running a 19 1/2 Duckworth with 115 Yamaha on a pod so our boats may be similar.
GLG

 

[JAC]

I have done a lot of research on this issue. I had this problem on my Hewescraft, custom weld and stabicraft. It's called porpoising. The first thing you should check is that you engine is not mounted to low. On all of my boats I have raised the motor one or two holes then the dealer setup. If the motor is to low it will make your max rpm lower the use of the full motor trim will be less and the boat will propose plus you will gain top end if you raise the engine. The next thing to try is a hydrofoil which is cheaper then trim tabs. Hydrolic trim tabs are the best but you could also use cheaper smart tabs to help solve the problem. Also on a lot of aluminum boats they will Bend the last inch of the hull on the at the transom to help solve this problem.

 

[Bigman]

Thanks for the comments and opinions all, very much appreciated.
I don't have much leeway as far as weight distribution is concerned, there's not much weight to move around. The batteries and fuel tank are fixed in their spots; I'm 320 pounds and it did the same porpoising whether I was at the helm up front or at the back sitting on the transom. I'll need to make a few more trips and try a few more things befored I come to a decision on this. I hadn't thought about a different prop or moving the motor up a bit on the pod.

GLG, your Duckworth will be very much the same as my Thunderjet; a light boat with most of the weight out the back end on the pod.

Kildonan, you and Pat are more than just ballast but you may have to sit up in the bow on the run out to see Jeff in Bamfield...

More trips anad research I guess.
Thanks again,
B.

 

[scott craven]

Although the tabs are probably the best option the Doel fins do work.
I have had them on my boats for years and they help stop the porpoising
as well as stop the bow rising during hole shot.

 

[NautiGirl]

Same as what Craven says, but all the advice here is good. Personally, I started with weight distribution. I installed a second battery for my electronics up front, installed a SE300 fin on the motor ( stern lift and hole shot improved), then played with props and found a slightly larger diameter worked well. The size and shape of the waves decides if I'll start to porpoise and then I just play with my speed and trim to stop it. Also, if I have no fishing partner I bring along my halibut anchoring set up and put that in the bow/port side seat area.