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Author Topic:   props vs. rpm
brendaringo posted 05-04-2003 09:54 AM ET (US)   Profile for brendaringo   Send Email to brendaringo  
I recently purchased a 1955 Robalo 18' center console boat with a 150 Mariner. I find the boat is slow to plan and my top speed is 37mph @ 5000rpms. The motor is in excellent condition as well as the boat. I recently traveled about 45 miles and used 45 gallons of gas. Should I be turning a higher rpm and a greater mph. It feels that my boat is to heavy or my engine is under powered. I've been told the engine is more than enough to power my boat. This boat weights about 2800lbs. I need some advice.

Brenda Ringo

jimh posted 05-04-2003 10:37 AM ET (US)     Profile for jimh  Send Email to jimh     

The owner's manual of your engine will be the most accurate source of the recommended maximum crankcase speed for your particular engine.

You may also find reading the several articles on propellers and their selection in the reference section to be beneficial.

Your figure of 1-MPG sounds like excessive fuel consumption, but depending on the speed at which you were operating your engine it is not beyond the normal range of fuel economy for a 2-stroke outboard of conventional design. About 2-MPG is the best they can do, and sea state and speed affect the fuel economy.

You are welcome to use the Propeller Calculator accessory on this website to help you find the best prop.

The purpose of this website is to collect and provide information on Boston Whaler boats, concentrating on the "classic" models made from 1958 to c.1995 in particular.

Questions about older Robolo boats are outside of the range of our topic and interests, and inclusion of such questions tends to pollute and reduce the value of the information we are collecting and providing here.

You are welcome to read the many threads which exist on the topic of repowering and propeller selection with respect to Boston Whaler boats, much of which will be directly applicable to your situation.

This thread will not be archived.


jimh posted 05-04-2003 11:07 AM ET (US)     Profile for jimh  Send Email to jimh     
[Administrative post]
Bigshot posted 05-09-2003 04:59 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
It depends on what 150 Mariner you own. I believe it should be 5500 redline but could be 57-5800. I would try a smaller wheel, maybe just diameter, not pitch depending on what you are running. Where did you find a 1955 Robalo :)

robalos are not light so don't expect much more than low 40's.

wayne baker posted 05-09-2003 08:07 PM ET (US)     Profile for wayne baker  Send Email to wayne baker     
I have had several v 6 mercs. consult your owners guide as suggested by others but I would think turn her up to about 5800 rpm's at wide open throttle.
Sal DiMercurio posted 05-09-2003 08:34 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Brenda, you don't say what size prop your running on that engine.
Off the top of my head i'd say your engine is rated to 5,600 max, which means your running 500 rpms under max, but much better to allow that engine to be able to reach that 5,600 rpms.
Your engine will run much easier if allow to reach it's max rpms.
Think of it this way, if your towing your boat on flat land in your truck or car, & your in 4th gear [ stick shift ] & you come to a fairly steep hill but you don't down shift, pretty soon the truck is going to be going 35 mph from 55 mph & your foot is to the floor on the gas pedal & your rpms are only 1,500.......your engine is straining to get some rpms back but can't because your in the wrong gear, same as a boat,........let it be able to get to it's max rpm range & you will use less fuel & possibly more top end speed.
Your engione will be very happy if you let it do this, but if you continue to lug it, she's gone say the hell with you & start to give you probllems much sooner then she should.
You need to run your engine wide open & trim it out as much as possible [ if you have tilt & trim ] & check your tach & see what rpms your twisting.
You wont get much of an rpm change by changing the diameter of your prop, "at the very most" maybe 75 - 100 rpms per inch maximum, it's the pitch that determines your rpm range, that 150 shouldn't be running anything bigger then a 14.25 diameter & i'll guess a 17 pitch prop.
Without knowing what size prop your turning, we can't tell you exactly which size will be correct for your application until you fill in the blanks.

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