motor trim setup

[shawn mcghee]

Guys, i have a 3.95 bermuda with a 25hp yammie,when going full tilt the boat tends to porpoise, any ideas how to get a better ride.

[madaff]

The boat can porpoise for a couple of reasons. sometimes its weight distribution, and the other is trim. I dare say it will be a case of both. Chances are you have your trim in the deepest position, this causes the thrust from the motor to push your bow down which can cause this effect. If you look at the back of your motor where it bolts to the transom you will see up to 5 holes on a bracket, if your boat porpoises you have it in the bottom or closest hole.

Now before you put the boat in the water next trip lift the leg of the motor and remove the pin and set it in the next hole (shallower position). It will make your bow rider higher but should take care of the porpoising. It may even give you better top end speed.

[Stu_000]

Shawn

I had the same issue on my punt 3.5m with a 15hp two stroke. I found that having the trim set hard against the transom that the porpoising would stop. If I trimmed it one notch up it would start again. This tended to happen at higher speeds. I realise we have a different setup but I guess the theory is the same. I was told by a reputable prop dealer that to get best performance it was better to trim it as above for a small vessel such as mine.

The other thing he told me was to jack the motor up with a block of wood or similar to get the prop closer to the water surface (but not too high to avoid cavitation). The height to lift would be dependant on how deep your prop is atm. After doing this I got an extra 2 knots out my rig.

Also I have heard a outboard "foil" may assist with this issue. I seem to recall a thread about 18 months - 2 years ago. A particular hull (Hornet?) was having the same issue as you and the foil seemed to help. Try a search and you may come across more info on the topic.

So I guess you have to experiment a bit with your boat to see what works for your setup. You may find you sacrifice some speed for a smoother ride but then again a smoother ride may help achieve more speed

Hope this helps

[Stu_000]

Here's that thread I was talking about

Hornets

from July 2005. Seems like there is a bit of conflicting opinion on that thread as per usual .

To be honest I always thought a perma-trim/foil was to get the boat on to the plane quicker or to at least assist a boat to plane.

[shawn mcghee]

Thanks madaff and stu, i had the motor sitting on the second hole from the transom, ihave now moved it to the third to see if it improves or gets worse.The motor already has a foil fitted,which i may remove until i can sort the other issue out then put it on later if needed

[Stu_000]

I would try it hard up on the transom in the first hole as well. Leave the foil on until you find out if you can sort the problem by trim alone. If not take it off and try again.

Good luck

BTW post up how it all goes. I am interested for future reference

[Rosie]

Try it hard up against the Transom Next time mate, should halp things alot

[shawn mcghee]

After checking a couple of things suggested i have found that the prop was sitting about 8 to 10 inches(200 to 250mm) below the bottom of the hull.I have lifted it around 1 1/2inches(35 to 40mm).And have also put the pin back to he closest hole to the transom,i hope to take her for a run in the morning and will let you know of the results.

[shawn mcghee]

here are a couple of photos of the motor setup as high as it goes.I dont know if it is OK where it is but will test it over the weekend to see how it goes.

[shawn mcghee]

another

[Rossco32]

Too low if you ask me.

Cav plate level with transom is a good starting point.

Those upswept foils can cause more handling issues than they fix..... Like rolling the boat outwards in planing turns.....

My 400 hornet (Merc 40 2 stroke) had an alloy plate that extended behind the prop, Never had porposing issues with that hull and no handling issues.