Low RPMs in reverse

[raevin51]

I was asked a question by my son this weekend and could not answer it. He has a 1983 Bayliner Capri Cuddy cabin with a Johnson 235 for power on it. he is running a 4 blade stainless prop 15 1/2 X 21p. the engine apears to be under propped, it can turn about 7000 rpm in forward. The question was and is is it normal for this type of prop to restrict RPMs in reverse? he can only bring the engine up to about 1200 RPM in reverse and it seems to have very little reverse thrust. Any ideas about this?

 

[steelespike]

Re: Low RPMs in reverse

My guess would be a throttle adjustment problem.It would have very little thrust at 1200 rpm.What size is the boat ?that is a lot of motor.What year motor?Motor is probably rated at 5,250 rpm I wouldn't exceed 5,500 unless you know the correct rpm range. Your boat is either very light or your tach is wrong.

 

[raevin51]

Re: Low RPMs in reverse

I will have him check the adjustment, that was my thought on the problem too. The boat is an 18 foot bayliner and yes that is a lot of motor for it, it clocks out in excess of 60 mph on the gps. I believe the motor is late 80's early 90's not really sure.

 

[QC]

Re: Low RPMs in reverse

I believe that steelespike is correct. And I would be watching that transom really close . . .

A little side note, I have had two boats that the control handle travel was impeded in reverse by impact with my seat . . .

 

[Dhadley]

Re: Low RPMs in reverse

I'd check the tach. It's doubtful a 235 will turn 7000, especially if it has stock reeds.

 

[dlkiller]

Re: Low RPMs in reverse

I'm not familiar with your boat, but I believe most controls restrict engine speed in reverse to that level for safety reasons. Crank that engine wide open in reverse and you'll probably get a swimming lesson. The prop has very little thrust because the exhaust exits through the hub, so in reverse the prop blades are pumping air, not just water.

 

[Silvertip]

Re: Low RPMs in reverse

Prop blades are also designed for optimum forward thrust, not reverse.