trim and speed question

Hello,

I have a 1988 18' bowrider with a 3.0 mercruiser and an alpha 1 gen 1 drive.

When trying to trim my boat, I gain no RPM or speed.

The motor turns about 4200-4300 WOT at about 35 MPH.

When I am trimmed down idleing out of the no wake zone and go full throttle,
it jumps to plane no problem, I then try to trim up and I gain nothing.

If I trim to far, it starts porposing on me.

Aren't you suppose to gain RPM or speed when trimmed up some?

Even trimmed down the boat rides good and is close to level on plane.

any suggestions?

Thank you,

wncrjb

Re: trim and speed question

That might be really cool not a bad thing.

If your boat has the x demensin right i.e. height of your outdrive right for your boat you really shouldn't have to play with it.

My brother has a velocity that when the motors are set level and it's on plane playing with the trim will only drop off a couple mph on the gps and maybe make for a cooler rooster tail tail but that's it.

Just saying might not be bad might be a real nice boat.

Re: trim and speed question

Well I am wondering what you "feel", see and hear. Does the bow come up at all with positive (our or up) trim? Can you hear or see the splash from the bow going further aft? I have never been in a boat that didn't gain speed with positive trim. The porpoising would indicate otherwise, but I am wondering if you have a lot of weight in the bow? Also, at 35 MPH trim is less effective than at 45, but I still think you should gain some. Have you ran the boat with a VERY light load? Low on fuel, no passengers, no ice chest?

Re: trim and speed question

My 86 18' OMC 3.0L would gain about 5MPH by myself. It would only gain about 2 with 2 adults and 3 childern. In other words, the more people I had less speed was gained by trimming, but I always had more control.

Re: trim and speed question

wncrjb, You may not see any RPM gain when you trim, but your speed should increase a bit. Some boats do not have too much advantage due to trimming, however, I would think your boat should show a bit of advantage. Does the trim really work? Does it trim enough to make a difference?

Watch to see where the water sprays off the hull when trimmed in. Now see if it moves toward the transom, as you trim up. Eventually you should hear the prop lose bite. You may need to use the trailer button to trim up enough to cause this.

Re: trim and speed question

QC said:

Well I am wondering what you "feel", see and hear. Does the bow come up at all with positive (our or up) trim? Can you hear or see the splash from the bow going further aft? I have never been in a boat that didn't gain speed with positive trim. The porpoising would indicate otherwise, but I am wondering if you have a lot of weight in the bow? Also, at 35 MPH trim is less effective than at 45, but I still think you should gain some. Have you ran the boat with a VERY light load? Low on fuel, no passengers, no ice chest?

Click to expand...

Well QC, to answer that one is easy.....
When going from idle to WOT, the bow rises for a few seconds only, then it levels out ( on plane ).
I have tried it with just me in the boat ( 250 pounds ) and tried it with as many as 5 in the boat ( close to max weight ), even tried moving people around in the boat and the results are very similar. With the boat loaded, it takes only a second or so longer to get to plane.... Actually for a 3.0, it impresses me......

I know the trim does work and is adjusted to the specs in the manual.

While at plane, with it trimmed DOWN, I see water spray coming from the sides about on the rear third of the boat. When I begin to trim UP, the bow doesn't rise or lower ..... Again it doesn't seem to matter of how much weight or where I place the weight. All I feel is when trimmed too high, it will porpoise on me.

The only time I saw and felt a difference is when I screwed up and went from idle to WOT with the trim in the UP position... ( I don't have a trim gauge )I stuck the bow about straight up in the air ( cost me a good pair of underwear LOL )......

I know I'm new to the boating world so I'm sure I'm over-looking something.

Thanks for all your help

wncrjb

Re: trim and speed question

Does this boat have a Hydrofoil on it? Just want to eliminate from discussion. Trim tabs? Other than that it still seems bow heavy to me.

Re: trim and speed question

QC said:

Does this boat have a Hydrofoil on it? Just want to eliminate from discussion. Trim tabs? Other than that it still seems bow heavy to me.

Click to expand...

There are no trim tabs or hydrofoils or anything like that on it.

I always thought it was light in the bow......... ( I'm a rookie LOL )

When sitting still in the water, the bow sits higher than the stern, even with someone sitting in the bow.

Thanks for the input,
wncrjb

Re: trim and speed question

QC said:

Does this boat have a Hydrofoil on it? Just want to eliminate from discussion. Trim tabs? Other than that it still seems bow heavy to me.

Click to expand...

I am on the bow heavy mind set as well.

Re: trim and speed question

Well usually if they porpoise they are bow lite. But you said that nothing happens until it porpoises. For a test, you should ride with only one other passenger; put them in the stern. See if it porpoises earlier. Then move him/her to the bow and see if it porpoises later (higher in the trim range), or blows out (starts to rev up some). If it revs up, that would indicate that the bow is heavy with the passenger. Sometimes you can't control who is where, but at least you may know that the weight balance is reasonable depending on where you put people.

I want my boats setup so that at the very top of trim range they will just start to porpoise. One bump back on the trim and that should be the sweet spot for WOT. Your boat is a little slower than I am used to, but I still think trim should gain you a few MPH from full down to some higher trim setting. Maybe only 2 or 3 MPH AND some RPM (they almost always go hand in hand), but even at that speed my boats have all reacted positively to trim.

Re: trim and speed question

Thanks alot for the help....

Next time I go to the lake I will try your suggestions and let you know how I make out....

wncrjb

Re: trim and speed question

When you say propoise, you meen when the front of the boat bounces from being trimmed to high? My boat was doing that today (1st time out) and trimmed down a little and it seemed fine and fast.

Re: trim and speed question

I think the bow is too lite or it is already trimed too high. Reason is you said when your trim down is the only time you get spray off the back 1/3 of the hull. Normally your fastest trim will be with the back 1/3 of the hull in the water trimed all the was up. With bow trimed down where you will loose speed most of the hull is in the water and the spray comes off the bow. In big chop and swells this will give you a better ride.