Prop Questions

[Roccafeller]

Hey everyone....new(ish) to boating. I own a 70's Starcraft Tri-hull, with a 1983 70hp 2 stroke Mercury. I do not know if i have the right prop on the boat. With 2 people on i can get on plane relatively quickly. with 4 people the boat struggles to get up, feels heavy. Motor is rebuilt. i trim the engine which helps....but i cannot pull a tube as ithe boat wont plane

Wondering if i have the wrong prop...( and i still dont understand which pitch and size i would need anyway)

Does anyone know what my RPM at WOT should be for this engine? (i have to hook up the tach so i dont know what i am currently running at)

To add to that, i used GPS and with trim i can get to 28mph
Thanks

 

[GA_Boater]

Roccafeller

Moved from another member's thread.

Kept this in the prop section, but if you can get to 28 MPH and the boat won't plane, you have other issues.

 

[Roccafeller]

Sorry, to clarify...i can get the boat to plane with minimal issue with 2 people on it, and i got it to 28mph on plane. My concern is when i add another 2 people it seems sluggish. I am wondering if i have wrong prop and need to know WOT rpm. Would like to pull a tube but extra weight holds me back

 

[WesNewell]

From what I found, range for that engine is 4800-5200 rpm. Without knowing the dry weight of your boat, it's hard to say what prop would work best for you. Could be anywhere from 12-17p depending on what you use it for and how much weight you carry. With a normal load, you'd want the wot rpm to be ~5000rpm.

 

[Roccafeller]

Great. Ty for the info

 

[ajgraz]

What prop you got on there now?

 

[Roccafeller]

No idea. Don't know much about props and pitch. All I know us it has 3 blades. Lol

 

[jimmbo]

In 1983, the WOT range of the 70hp engine was 5300 - 5800
Gear Ratio was 1.83:1 or 22:12

 

[Roccafeller]

thank you... for general overall performance...fishing, tubing touring etc...any suggestions on what size prop and pitch i should be looking at?

 

[David Young]

Check your compression of each cylinder. You can rent a gauge from your local auto parts store for 'Free'. I had a boat many years ago, ran good with 2 people but ran poor with 4 people. The motor was worn out .

 

[ajgraz]

Is that the 15’ TriStar? I owned one (1978) for a number of years, also with the 3cyl Merc 70.

Do check compression as suggested; I had to replace the power head as saltwater eventually ate through the baffle plate and destroyed a cylinder.

Bear in mind that 70hp is really a 50hp by today’s measuring standards, and that’s a heavy Hull for its size.

28mph is about as fast as mine ever went. Be good to know what your WOT rpms are.

I know you said you don’t know what prop is on it...but does it have any numbers on it? Is it aluminum? Stainless? Plastic?

Anyway, I tried probably 6 different props on mine over the years, and the only one I was ever truly happy with was a Comprop M4321, plastic 4-blade 11” pitch. It spun about the same rpm as the 13 pitch alum 3-blade Merc Black Max the motor came with (probably what’s on yours now), but had much better bite and hole shot, which is what it sounds like you need. Ran that sucker at least 5 years with no problem, so don’t discount the plastic!

 

[Roccafeller]

15ft starcraft capri. I think it is a '77. (no HIN). I will check motor. Apparently the motor was rebuilt a year before a bought it. Starts up with quick turn of the key. I will check the prop for numbers this weekend. I believe the current prop is aluminum

 

[ajgraz]

From what I can tell from brochures linked elsewhere on this forum, 77-78 is when StarCraft changed from “Capri” to TriStar, they look to be essentially the same.

Anyway, one more very important detail: I was also never happy until after I raised my Merc 1.5” on the transom. I rested the bracket on top of a chunk of 1.5” square aluminum extrusion.

 

[Roccafeller]

i will keep that in mind. I thank everyone for the information. I will get my tach hooked up and then go from there.