engine rpm vs speed

[husaburg560]

I have a 1982 21' bowrider with a 305 chevy mercruiser model 1 pre alpha. The engine rpm's will climb well past 4600. But anything over that the engine just revs and I do not gain any mph. The prop is 16" in diameter and 21 pitch. Am I correct thinking I need a larger pitch prop? Yes I had it trimmed out to. Thanks for any info.

 

[waterone1@aol.com]

Re: engine rpm vs speed

What speed are you getting ? I would first question if the prop hub is in good condition or if you are having some kind of prop cavitation or ventilation (I always get the two mixed up) problems. The correct rpm range for that engine is 4400 - 4800 rpm's. If you know of a local prop shop, you could approach them with your question and see if they have one for you to experiment with to get it right before buying. I would hate to advise you to go to a larger prop, only to find out your hub is slipping and you had the right size to begin with.

 

[3 Card]

Re: engine rpm vs speed

Sounds like a pretty big motor for the boat...I would expect you could spin a larger prop...<br /><br />I spin a 14X23 for cruising (95 Wellcraft 19' 4.3L V-6)<br /><br />Going to 23 pitch will bring your WOT down 300-400 RPMs...Assuming Waterone1's concern is not a problem...

 

[jlinder]

Re: engine rpm vs speed

3 card,<br />I am thinking the opposite. I drive a 1988 20' bowrider with the Chevy 5.0L (305 cu in) with the OMC drive. I run a 14.5" x 19 pitch prop and run 3900 WOT (changing to a 14.5x17 to get proper WOT). husaburg560 is running a 16x21 prop and over reving.<br /><br />I agree with waterone1. Sounds like something is slipping. Especially since it will go well over 4600 but get no increase in speed.<br /><br />Prop hub would be my first thought.

 

[3 Card]

Re: engine rpm vs speed

Jack,<br /><br />I was thinking Overrevs...go to bigger prop to bring it back in line...The slipping issue I have never run into before.<br /><br />Your situation sounds like under the range you need and going to a 17 would increase your RPMs @WOT...Isn't that the exact opposite of husaburg's problem?

 

[Doug Durako]

Re: engine rpm vs speed

Husaburg----Can't imagine that your prop would be 16 inches in diameter, unless it is from a pontoon boat.<br /><br />Get to a prop shop and have them take a look. Is this a new problem, or has it always run this way?<br /><br />Does it over-rev when you are trying to get on plane, or any other time in the power band?

 

[husaburg560]

Re: engine rpm vs speed

I will mark the prop hub with a bright yellow paint marker before I take the boat out again, to see if the hub is slipping. The boat is a v - hull and I have never got it over 33 mph. Thanks for the quick replies.

 

[husaburg560]

Re: engine rpm vs speed

I have always thought I had a problem with my top speed. The prop is off of another bowrider. To find the diameter don't you measure one of the lenghs of the blade and double it? Thats what I did and came up with 8".

 

[jlinder]

Re: engine rpm vs speed

3 Card - you are right. My WOT @ 3900 is under the 4200 - 4600 it should be. I have a 19 pitch prop now. Going to a 17 to get WOT where it should be.<br /><br />My comment was that if I am running the 14.5x19 prop with the same engine and comperable boat and only getting 3900, then a 16x21 turning way over 4600 does not sound right. The only thought I had was that the prop was slipping.

 

[jlinder]

Re: engine rpm vs speed

husaburg560,<br /><br />If you have slipping of the prop, you will not get the top speed you want. If you can borrow a prop try it. Let them suggest the size.

 

[husaburg560]

Re: engine rpm vs speed

I'll give that a try and thanks for the great suggestions.

 

[husaburg560]

Re: engine rpm vs speed

One more thing JackL, On your boat running WOT whats your mph?

 

[jlinder]

Re: engine rpm vs speed

About 36

 

[3 Card]

Re: engine rpm vs speed

Jack<br /><br />Got it...Doesn't pass the logic test...Leads it towards slipping prop<br /><br />Probably does make sense to borrow a prop and try it again...

 

[jlinder]

Re: engine rpm vs speed

husaburg,<br /><br />Would you please let us know how this comes out? <br /><br />It is alwyas great feedback (and a way to contribute back to the knowledge pool) to know what worked in real life.

 

[Richard Petersen]

Re: engine rpm vs speed

Go to a good prop shop and they will save you time, money and agravation.

 

[jlinder]

Re: engine rpm vs speed

husaburg,<br /><br />Any updates? Hope you got out this past weekend since the weather was great. (Your profile shows we are in the same city)

 

[husaburg560]

Re: engine rpm vs speed

Sorry about the delay, just got my compututer fixed. My prop hub was slipping, causing my top end problem. Thanks everyone.

 

[husaburg560]

Re: engine rpm vs speed

What lake do you normally go to Jack L? I go to Jackson lake.

 

[jlinder]

Re: engine rpm vs speed

Alatoona (sp?). Redtop mountain area, next to the bridge. <br /><br />Nice area, you can find a lot of areas for tubing or coves to pull into, anchor, and swim.<br /><br />For fun to to Tugaloo state park on lake Hartwell. For $80 a night you can rent a nice well equiped cabin with a boat dock. Real nice.