Prop pitch for underpowered boat

[Jimmyb617]

I have a 1978 fiberglass 17 ft cc wellcraft, bought a used 70 Johnson 2 stroke for it and it only is getting to 3600 rpms and 17 mph. Tested everything on the motor and everything runs good with it. Talked to a couple people and came to the conclusion that the boat is too heavy for that motor. It is an old boat had may be slightly waterlogged. The prop on it right now is. 13 3/4 x 15. if I decrease the pitch on the prop to get more rpms would t really increase it or does it matter because it's underpowered anyway. What would be a good size to try?

 

[jimmbo]

A 17 ft with a 70 should do between 25 and 30 mph, maybe a bit more. What rpms are you getting with the 15" prop? I suspect the engine is putting out less than advertised horse power.When you say "tested everything on the motor", please explain what everything is.
The water logging might be an issue, but throwing more power doesn't solve it.

 

[Jimmyb617]

A 17 ft with a 70 should do between 25 and 30 mph, maybe a bit more. What rpms are you getting with the 15" prop? I suspect the engine is putting out less than advertised horse power.When you say "tested everything on the motor", please explain what everything is.
The water logging might be an issue, but throwing more power doesn't solve it.

WOT is at 3500 rpms with the current prop. I've tested compression,fuel, spark, ignition system, only other thing someone suggested is to test the timing at max rpms but I neee to get my hands on a timing gun

 

[Jimmyb617]

Also rebuilt carbs with carb kits

 

[Sea Rider]

Have you checked if current prop doesn't have a spun hub, it's waterlogged or runs a combination of both ? With a heavy boat and a under powered OB, a less pitch is the way to go along carrying less passengers or load than if counting with max allowed HP.

Happy Boating

 

[Jimmyb617]

Have you checked if current prop doesn't have a spun hub, it's waterlogged or runs a combination of both ? With a heavy boat and a under powered OB, a less pitch is the way to go along carrying less passengers or load than if counting with max allowed HP.

Happy Boating

10 4 that's what I was thinking. The motor ran fine when it was on my buddy's boat he has a 16 ft aluminum.

 

[Scott Danforth]

Agreed, something not right with the motor if your only getting 3600 RPM. is the boat even getting on plane, or is the bow still pointed skyward?

 

[Jimmyb617]

Agreed, something not right with the motor if your only getting 3600 RPM. is the boat even getting on plane, or is the bow still pointed skyward?

No it doesn't get on plane

 

[Jimmyb617]

I really don't know what else to check. I also checked the timer base to make sure it was moving correctly

 

[jimmbo]

When your are checking things, when the throttle lever is at full power. are the carb throttle valves opening fully?

 

[Scott Danforth]

Are you trimmed in all the way? do you have a cooler or other items such as an anchor you can move toward the bow?

is the vent open on the fuel tank?

 

[mickyryan]

im going for waterlogged boat imho because he should be able to hit plane at 3500 rpm eventually take boat to a weight station and weigh it then take trailer separetly compare to original weight and that will tell you if weight is a issue.

 

[Chris1956]

If the boat is not planing at 3600RPM, something is wacky. Boats will normally plane between 2000 and 2500 RPM. Is your tach accurate?


The motor must be trimmed all the way down, or the trim lock rod set in the first or second hole, from the hull.

Not sure what else to suggest. Maybe the hull has a convex or hook in it? Maybe the transom flexes under power?

 

[JCurry2885]

I have almost the same setup you have. 17'6" 89 vip boat. 1977 Johnson 70hp. Also have the same 15 pitch prop you have. Mine is a little slow, but does get on plane, and with 4 passengers will do 30mph according to the new faria speedometer onboard. I have my trim rod set to the middle notch on mine. My boat was really sluggish and would only do about 3k rpms before when my flywheel key had sheared, and I was not getting full timing advance at WOT...may be something to check.

 

[fhhuber]

Have you checked if current prop doesn't have a spun hub, it's waterlogged or runs a combination of both ? With a heavy boat and a under powered OB, a less pitch is the way to go along carrying less passengers or load than if counting with max allowed HP.

Happy Boating

Spun hub primary symptom is high rpm. Can't be a spun hub if he can't get up to normal WOT RPM.

It could be a bad engine alignment (and/or excess weight up front) plowing the nose of the boat down and preventing it from getting up to speed in normal planing angle.

 

[Sea Rider]

Spun hub primary symptom is high rpm. Can't be a spun hub if he can't get up to normal WOT RPM.

Don't agree with that, a high rpm will be noted if the vulcanized spun is completely shot, dead, kaputt, that's if being old, dried, cracked or just hit a rock. A good cond prop that's begining to experience a initial spun hub won't possibly make for OB to run at max wot rpm range. A hub slip test will confirm that issue..

Haappy Boating

 

[fhhuber]

But... if the prop is just SLIGHTLY slipping... its not going to cause the symptoms he's got, #1 being the engine no longer can get to normal WOT RPM. The engine just does not lose RPM simply due to the hub slipping. Something else MUST be going on.