Prop-Shoot

[CaptAhab]

I titled the thread Prop-Shoot cause it's a lot like a crap-shoot, choosin a prop.
My situation....18 foot aluminum boat 600 to 700 lbs bare very shallow v, max rec. hp 40..a big rowboat basically with a 1991 2-stroke 90 merc. The motor came with a 13.25x21 aluminum prop, hole shot was amazing, jumped out of the water...45 mph on the speedo 5300 rpms on the tach...sounds good eh? Don't think so...this motor can push 5800 for sure but the hull runs very wet and any positive trim blows out the prop,in turns forget it! With the quick hole-shot and fast acceleration I'm thinking this prop hasn't got what it takes to make any speed and now I get to the point, I want a prop for speed and this 21 aluminum prop I can slap on for heavy loads. I have already bid on e-bay for a prop a LazerII 13.5x24 with hub for $85 u.s. did I just make a mistake?

 

[Randybeall]

Re: Prop-Shoot

There is a science to selcting the correct prop for an application. If I under stand right you have a boat rated for 40 horsepower with a 90 horsepower engine on it. I don't believe speedometers anymore, seen them off too much both ways, at speed you have a hard time keeping the pitot in clear water. Get a GPS if you want to know speed. Wide open throttle only 5300 rpm, have you checked throttle plates to make sure they are exactly wide open when you put the throttle lever there. Where is the engine mounted, the anti ventilation plate should ride at the surface of the water when going straight on fairly smooth water. If the boat design won't ever plane you might look for a 4 blade prop to transfer more of the engines power to the water and go down in pitch, perhaps 19 inch pitch. Check every phase of engine tuning, if the horsepower is not there it will not make the top rpm. I would do this before changing any prop. Be Careful. That engine and boat combination is asking for an accident.

 

[CaptAhab]

Re: Prop-Shoot

Thanks for the reply! As you can see in my avatar the motor is set pretty normal a 20 inch transom and a 20 inch lower end, motor is mounted third hole from the bottom and the ventilation -cavitation plate? is even or slightly above the keel line, I think you'r right about the speedo,feels more like 40 mph at 5500 rpm, the boat planes in about 2 seconds and under speed the front is down on the water.Trim up is very limited due to prop slip, moved the motor up and down experimenting but still the prop slip.Otherwise the boat handles pretty good as long as you trim down in turns and take it easy on windy days I wouldn't let an inexperienced person drive it though...

 

[steelespike]

Re: Prop-Shoot

I think perhaps a stainless or Hustler turning point may solve your venting problem and let you maintain your height and still trim more.
That boat/motor combo is asking for trouble.Watch your transom and keep in mind there could be legal problems in the event someone gets hurt.

 

[CaptAhab]

Re: Prop-Shoot

Thanks for the reply,I'm glad that someone agrees with me that this prop is not ideal for this set-up and I am quite impressed with the laserII especially after I watched a vid on you tube of a 100hp Merc pushing a 24 pitch Laser on a 16.5 runabout, awesome! link {http://www.youtube.com/watch?v=sdtGUNztTjk} or type this in the youtube searchbox { 100HP MERCURY 90' with 24" Laser2 prop}
Easy size to find used online..

 

[ivar]

Re: Prop-Shoot

I'd think 24" might be too much.. I'd look for a SS prop with the same pitch you got

 

[CaptAhab]

Re: Prop-Shoot

By golly I bet you'r right, Thanks for answering my original question, I won't bid any higher on that prop, I saw it cheap and figured if the hull was looser the 24 would start to work, like shifting gears at a certain trim level...if nobody overbids me I'm stuck with the 24 but I'm sure I could find use for it.

 

[CaptAhab]

Re: Prop-Shoot

Ive tried Mercury's Prop Selector on their page and it gives me me 21 ptch laser2 or 21 pitch trophy 4-blade, I was choosing the boat model as a aluminum center console so I switched to aluminum-tiller 15'-17' with rpm at 6000 and pitch at 21 and speed at 40 which I've done with the rev limiter disconnected while the prop was blowing out and the motor on the highest hole, then it gives me the Laser 2 13.5x24 pitch or the Trophy 23 pitch....so now I dunno..
What's the formula for speed 5600 rpm and 24 pitch prop I'm not sure what slippage I would have.. 55mph at 5500 rpm would be poetic.

 

[CaptAhab]

Re: Prop-Shoot

Ok got out-bid on the 24 pitch and landed a LaserII 22 pitch on E-Bay ! I kind of came to my senses and opted for the 22...now that should work pretty good.
I hope..

 

[CaptAhab]

Re: Prop-Shoot

Ok I got the the 22 pitch SS on for weekend fishing outing and I have to say it performed amazingly, with 3-200 lb. guys a whole lot of muskie gear and food we hit 41mph gps,this prop can be trimmed up a whole lot more then the stock aluminum, though it still blows out sooner then I would like or expect. Being my first aluminum boat I keep expecting it to act like a smooth,stiff, sharp edged glass-hull which it ain't..it has multiple hooks in the bottom.. and the keel curves up to meet the transom at the extreme trailing-edge of the boat... not very conducive to high speeds.
I dumped my bro's and fishing gear and took it out on some choppy water
and got it trimmed up good at about 40 and it kinda shifted gears and I hit 47mph before I had to slow down because of windy conditions.I think this prop might get me to 50mph in proper conditions and if so I am more then happy with my choice thanks to to the info I got here!
p.s. my speedo is totally in sync with the gps..