1985 Starcraft porpoising issue. Prop??

[derek4325]

So I bought the boat last year. It's an 18' aluminum with a 2.5l I/O mercruiser. Runs fine and is mechanically sound. Started with a 17p prop and I could get it to 4k RPM. If I trim any higher it starts porpoising. Tag on the motor says WOT is 3900- 4300 rpm, but I read that the spec was changed that same year to 4200- 4600 rpm, so I figured I drop to a 15p. That made it worse. At WOT it's porpoising at about 3600 as I trim up. Should I try the opposite and go up to a 19p? I can live with a max of 4,000 but I also don't want to lug the motor. Thoughts anyone? P.S. the boat has not been modified an any way and it's just me in it when I run the tests. Both props I used were aluminum, also there is no rot or waterlogged wood. Thank you.

 

[Scott Danforth]

how are you triming it? should only need to trim up just a tad after the holeshot. once it starts porpoising, tap the trim button once or twice to get it to settle down

you never want to lug a motor, so no, dont run a 19p.

I am assuming you weighted the boat to make sure you are not carrying extra water in the flotation foam,

put some weight in the bow. a cooler is usually enough to prevent it from porpoising.

 

[derek4325]

how are you triming it? should only need to trim up just a tad after the holeshot. once it starts porpoising, tap the trim button once or twice to get it to settle down

you never want to lug a motor, so no, dont run a 19p.

I am assuming you weighted the boat to make sure you are not carrying extra water in the flotation foam,

put some weight in the bow. a cooler is usually enough to prevent it from porpoising.

After it settles I am running around 3500- 3600 at WOT with a 15p. With a 17p I could go 4000. You're saying to keep the 15p and add weight in the front? Starcraft put everything in the back. Gas tank, battery. I've considered routing the battery to the front.

 

[Scott06]

After it settles I am running around 3500- 3600 at WOT with a 15p. With a 17p I could go 4000. You're saying to keep the 15p and add weight in the front? Starcraft put everything in the back. Gas tank, battery. I've considered routing the battery to the front.

The rpm numbers don’t add up as you go down in pitch wot rpm will be higher not lower unless prop is blowing out

Is the tach correct

Scott has pointed you correct trim up when it starts proposing trim back in or add weight to front

Were these different pitches of same prop ?

So not go up in pitch the 4 k was barely adequate

Neighbors of mine had same boat engine combo growing up

 

[derek4325]

The rpm numbers don’t add up as you go down in pitch wot rpm will be higher not lower unless prop is blowing out

Is the tach correct

Scott has pointed you correct trim up when it starts proposing trim back in or add weight to front

Were these different pitches of same prop ?

So not go up in pitch the 4 k was barely adequate

Neighbors of mine had same boat engine combo growing up

Rpm numbers don't add up because the boat will porpoise before I can trim all the way. As you and Scott mentioned I think I'm in a good place with the 15p prop, just a matter of pushing the front end down. Blue fin had a similar hull with an I/O and they put the gas tank in the front. I scratched my head over that but I think I know why they did it now... if I want out on longer trips more often I would just put a cooler up front, but I think I will relocate the battery for a more permanent solution. I thought about smart tabs but I really don't think that's necessary on an 18' aluminum runabout

 

[airshot]

I have always adjusted my prop pitch to give me max wot rpm with just me onboard, this way adding extra passengers and gear will keep my rpm in a safe range yet give me best speed. I always carried a spare prop with 2" less pitch for skiing and knee boarding than o get the hole shot. I would be testing your tach to be sure it reads correctly. Get on the ground and look eye level at your keel....it should be straight, no hook or curve along its length. Check your boats weight at local truck scale to be sure your not waterlogged. Proposing is usually caused by trimming the lower unit ( outdrive) to far out (away from the transom) causing the bow to rise up in the air. Trmming in to far usually causes the bow to bury into the sea. Trim in for your hole shot then bring your trim out a little at a time until the porpoise starts. Also check the props you have to be sure they haven't been reworked to a different pitch.

 

[derek4325]

I have always adjusted my prop pitch to give me max wot rpm with just me onboard, this way adding extra passengers and gear will keep my rpm in a safe range yet give me best speed. I always carried a spare prop with 2" less pitch for skiing and knee boarding than o get the hole shot. I would be testing your tach to be sure it reads correctly. Get on the ground and look eye level at your keel....it should be straight, no hook or curve along its length. Check your boats weight at local truck scale to be sure your not waterlogged. Proposing is usually caused by trimming the lower unit ( outdrive) to far out (away from the transom) causing the bow to rise up in the air. Trmming in to far usually causes the bow to bury into the sea. Trim in for your hole shot then bring your trim out a little at a time until the porpoise starts. Also check the props you have to be sure they haven't been reworked to a different pitch.

Tach is correct. Matches the tach setting on my timing light very close. Boat is definitely not waterlogged. Haven't checked the keel but I can't imagine that's an issue. I had it out yesterday and I could be over trimming like Scott suggested, but that's with a 15P. Should I really go to 13p or even smaller?? That seems really small. That being said it is aluminum and not very front heavy. That being said I follow the same procedure you do whenever I size a prop. Never really had an issue before

 

[Scott Danforth]

you should be between a 15P and 17P on the prop

how far up are you from full down?

 

[derek4325]

you should be between a 15P and 17P on the prop

how far up are you from full down?

Not sure exactly but I would say less than half. Yesterday I tried it out with my camper battery in the bow (approximately 70 lbs) and it helped a little but not nearly enough

 

[Scott Danforth]

Not sure exactly but I would say less than half.

I dont think i ever trimmed up that high with any Mercruiser . i think about 1/4 on the gauge is all that I ever did unless there were a lot of people up in the bow.

 

[redneck joe]

Two .0003 cents worth of thought is to agree with Scott's post. On my mercruiser I rarely had it trimmed up at all unless someone was in the cabin.

As a bow rider I'm assuming yours was designed to have people, aka weight, in the front so they have allowed for extra trim capability. With just you there is no need. Can't speak to the rpms. I'm not in anyway qualified to address that.

On my 18 MR it's mostly just me I many times forget to trim up and do not feel nose deep. but i also mostly never max my rpms. When I do trim, and on most all boats I've own in this size range, I shoot to have the spray coming out about where the helm is or slightly behind.

Put 250 lbs of people in the bow and give it a whirl.

 

[redneck joe]

Side note, good looking boat.

 

[airshot]

While I do consider them a last effort, you might consider adding a tail fin on the cavitation plate on your outdrive. I have saw them solve such issues. Being a toolmaker, I have always made my own from SS sheetmetal, they worked much better and were smaller than the big plastic contraptions sold on the market.

 

[derek4325]

I dont think i ever trimmed up that high with any Mercruiser . i think about 1/4 on the gauge is all that I ever did unless there were a lot of people up in the bow.

I'm sure I'm at least 1/4. But not exactly sure where I begin to go too much. Trim gage doesn't work

 

[Scott Danforth]

No gauge needed

When on the hole shot, drive all the way down, as you mash the throttle and the boat comes out of the hole and starts to settle down......thumb up on the trim until you start to get porpoising. Than tap it down a bit until it settles. You will feel the boat pick up speed

If you change directions, you may need to adjust depending on wind and wave action

When you back off the throttle, thumb down on the trim

 

[Pmt133]

I agree with Scott. No gauge needed. My boat kind of tells you when its happy... Tucked in hammer down. Bow drops start tapping up on the trim. The steering will loosen up first followed by a 2-400 rpm jump when you hit the sweet spot. Water off the sides is typically near the last third of the boat and the wake behind goes from a concentrated jet look to a nice flat fan.

 

[redneck joe]

Yeah, forgot about steering feel .