prop recommendations

I have a Aq125 / 270 in a 17.5 ' double eagle, very heavy boat purchased it three years ago finally got her in the water
The problem it takes me half the lake and all morning with the girl friend paddling to get her to plane the top speed is 26 mph at 3000 rpm. I am ok with 26 mph its the holeshot that doesn't exist.
I am suspecting I have the wrong prop I cannot find any markings on the prop to determine pitch, the prop diam is 13". The engine is fresh, new 2bbl roch carb and pertronix ign, runs like a top. Volvo specs; max rpm 5100 cruising about 4400
any ideas.
pete.

Re: prop recommendations

Take it to a prop shop and let them measure the pitch for you.

Mark

Re: prop recommendations

Texasmark said:

Take it to a prop shop and let them measure the pitch for you.

Mark

Click to expand...

Dropped the prop of on the weekend just talked to the prop guru today its a Michigan old prop very hard to cross reference he suspects its a 13" pitch, prop diam is 13".

Re: prop recommendations

Know your outdrive gear ratio? Looking to do some performance specks for you and need that tidbit to go with the 5100 rpms assumed to be at WOT.

Mark

Re: prop recommendations

The AQ 270b which I have is 2.15;1 that's as low as they go. I have a 15x11" pitch prop coming I will give it a try , my gut feeling is this boat is too heavy for the engine displacement. I plan on taking it out as soon as the new prop shows up, will also take some propane with me and give her a shot at wide open throttle if its lean , I will re-jet a little richer if that's the case. I picked up the old prop today as close as we could make out the prop is a 14x12", not going to gain a lot if anything with the 15x11" but who knows might be surprised. My last resort is to start looking at engine options , not sure what would bolt up, maybe a 4.3.
Thanks Mark
Pete.

Re: prop recommendations

Ran some quick numbers through the go-fast.com, BAM prop slip calculator:

I took your 2.15 and your 26mph and 3000 rpms and with the estimated prop shop numbers of 13P it gave me 17 mph with NO slip which is impossible. Don't know where the 26 mph you mentioned came from unless your tach or your speedo is miserably wrong. For a lot of boats, 17 is at, just over, or just under the planing speed. Not a good place to be.

However if your engine was in fact rolling out the 5100 rpms with no slip the estimation is 29 mph. Since slip is always there and you said you were heavy, I used 20% for slip giving you a 23 mph top end.

Don't understand your comment about got the old prop....aka what old prop, the one the prop shop said was 13? Anyway, running 14x11 would be between the first number and the next prop:

15x11 shows to be 25mph at 5100 NS and since you have a better thrusting prop, I assumed slip would drop to 15% and actual speed would be 21. Also, 4 blades over 3 work better in a work environment....getting on plane, but increased diameter and shallower pitch does too. Still, as you noted, whatever you hang on it requires torque x rpm to drive it.

However all this is pure speculation. Once you satisfy some engine torque/hp/load requirements things can change drastically as you noted; like getting above plane the boat drag drops significantly, the engine unloads so it needs a lower percentage of hp for the speed so it can move up in rpms which increases speed, usually decreases wetted area, thus drag, etc. Only way you know is to run it.

Mark

Re: prop recommendations

Interesting Mark, a good friend/mech also mentioned the rpms seemed unrealistic verses the boat speed, I will confirm with my digital tach next run. Boat speed is definitely correct , I'm using my Lowrance I finder GPS, have checked it against my truck speedo.
As far as the prop, yes my original prop we suspect by the Michigan numbers is a 13x12" number appears to be SMC556 not totally sure about the middle digit. Talking to prop suppliers I don't believe a four blade is even an option. New prop should be showing up in a couple of days.
I am really curious now to confirm my rpm that will shed a different light.
Good thing I am persistent and have the patience of Jobe.
Sure appreciate your input,
Thanks Mark.

Re: prop recommendations

The slip calculator shows 5487 rpm using a 2.15 ratio with a 12 pitch, using actual speed of 26 mph at 10% slip.

It appears that you have a tachometer issue if it's only showing 3K rpm. A 15 X 11 should run about the same rpm as a 14 X 12, since you're just trading an inch of diameter for an inch of pitch.

The larger diameter should provide an improvement in acceleration to planing speed.

Re: prop recommendations

Hi Mark, finally got the boat in the water, made the first run with the old prop with tach hooked up, the digital tach I took was a piece of junk too not very stable but I believe the average RPM reading was around 4600 at 26 MPH. I put the new 15 x11 prop on, wow what a differance was very pleased with the acceleration, but of course speed suffered I could only hit 21 mph and the engine was maxed out at 5000 RPM, still not where I want to be.
Would be happy with top speed of 26 mph if I can retain the holeshot. I am hoping your calculating tool can come up with a suggestion for the ultimate prop.
Looking at a Volvo chart my prop options are, 14 x 13, 14 x 15, 14 x 17, 14 x 19, 14 x 21, or 15 x 11 which I have 15 x 13, 15 x 15, and 15 x 17.
Look forward to hearing from you Mark.
Pete.

Re: prop recommendations

I would be looking at 14x15, you hole shot may have suffered because your old propeller was so beat up. I can't imagine a 125 not being able to push a 15 pitch with 2.15 ratio.

Is your boat waterlogged? No matter what you do, a 17.5' boat can't be that heavy.

Re: prop recommendations

Hi 500, thanks for your reply, I agree I think the condition and perhalps the design of the old prop is the root of my problem.
What are your thoughts or recommendations with dia and pitch, the 15 x 13 or the 14 x 15 or will they produce the same results ?
I am not sure if your familar with the Double Eagle boat , they are built in B.C. the boats that were built pre 1980 had a ton of fiberglass, very heavy and indestructable, Fisheries and Coast Guard used them back in the day.