Prop advice? 40hp Merc is SLOOOOWW

[misterguru]

//update - I checked and am running 4300 RPM at 19mph at WOT// <br /><br />I have a 15 foot fiberglass deep semi-V hull with a LOT of plywood which equals a lot of weight. <br /> <br />The hull was originally bare, but has about 3 sheets of 3/4" plywood for decks, 3-4 eight foot treated 2x6s, light carpeting, 12 gallon gas tank, 40# trolling motor, outboard/steering console, 40 lbs of tackle, etc... <br /> <br />My 1977 40hp Mercury has a 13 degree aluminum prop and with me and my daughter it will run 20 mph on the GPS. Add my wife and my other daughter... 18mph, and last night I added a friend and his camera equipment (fireworks pics) - about another 350-400 lbs, and 13.5 mph was tops. <br /> <br />I don't think I have any trouble with my "holeshot" but it makes sense to me to drop down in pitch to get RPMs up.... but the boat does seem to take off well, even very heavily loaded. So maybe I am confused and should go with more pitch... <br /> <br />I do not have a tach... sorry for lack of info. Is there an obvious direction to go here?

 

[fishingdan]

Re: Prop advice? 40hp Merc is SLOOOOWW

You really need to know what the engine rpms are to fine tune prop selection. Based on your description, it sounds like the boat is performing as expected. <br /><br />3/4" plywood for decking is a lot of weight. I have 3/8" decking in mine. <br /><br />Selecting a prop, like everything else on a boat, is all about tradeoffs and compromise. A prop that gives you a good holeshot may reduce your top end speed and vice versa. <br /><br />Two recommendations: <br /><br />1) Go to a good marina or prop shop and let them work with you to determine the actual weight of the boat (fully rigged with your typical load) and recommend a prop. Most good shops will lend you a prop for some testing. Try different props before you buy.<br /><br />2) Install a tach and then do #1. Regardless of the prop you select, it is important that your engine operates within the recommended rpm ranges at wide open throttle (WOT). Too high or low and you will reduce the life of your engine. <br /><br />For a point of reference, I have the 16' boat pictured. With 1 adult and 1 12 year old child, top speed is 26 mph. With 2 adults and 2 12 year old kids, my top speed is about 22 mph. With 2 adults, 5 kids (ages from 12 to twin boys that are 4) the boat tops out at 17-18 mph. I have a 1985 Merc 50 that has many hours on it. <br /><br />Good luck.

 

[phatmanmike]

Re: Prop advice? 40hp Merc is SLOOOOWW

Is there an obvious direction to go here?

yes, lose all that extra weight. whay all the wood? is this an ARK?

 

[misterguru]

Re: Prop advice? 40hp Merc is SLOOOOWW

lose all that extra weight. whay all the wood? is this an ARK?

Well, originally I didn't think the extra weight would make much of a difference.<br /><br />Even if I went to 3/8 plywood... that's half the thickness, so, ~100 lbs for the 3 sheets. Now that I have carpeting, bench seat and hatches all cut it would not be worth the effort to change it all out. Not to mention the 3/4" is a very stable floor.<br /><br />Oh yeah... I have 3 batteries in there too, two deep cycle for trolling and the one smaller one for starting.<br /><br />The old aluminum prop has been banged around a couble of rocks in its years, so if I can get a few more MPH or better mileage by dropping to 9, 10 or 11 degrees, or upping it to 15+, then I may as well.

 

[Laotzu]

Re: Prop advice? 40hp Merc is SLOOOOWW

I have a 91'50 on a 16 ft. Crestliner aluminum and get about the same mph under the conditions noted by fishingdan. Takes a long time to build up the speed tho with 7 adults in the boat, but still gets up to 20mph. One thing I did which helped was to remove the hydrafoil. Also improved my hpg.<br /><br />Jerry

 

[Moody Blue]

Re: Prop advice? 40hp Merc is SLOOOOWW

T_I_D that sounds like ALOT of boat (read heavy) for that motor. Without a tach you are only guessing what the resulting RPM's will be by changing the prop. By going to a higher pitch (15" not degree) you will gain a little more speed, hole shot will suffer, fuel consumption will decrease and RPM's will decrease. Swap out for an 11" pitch and hole shot will improve, top speed will decrease, fuel consumption will increase and RPM's will increase. 2 cycle motors like to run in their upper RPM range but you run the risk of over revving the motor at some point. <br />How is the motor trim angle set-up ? Is the boat planing at these speeds ?

 

[misterguru]

Re: Prop advice? 40hp Merc is SLOOOOWW

I bought a tach... I have to find some wiring info on the motor now.<br /><br />1977 402 40HP is 4 pulses per rev, right?

 

[misterguru]

Re: Prop advice? 40hp Merc is SLOOOOWW

OK, well I am traveling 19 mph today at 4300 when at WOT. The info I gather says my 40 HP should run about 5200 rpm... so 900 rpm too little... I should go to a 9" prop ( I am at 13 now) right?<br /><br />Any input?