How far down in pitch? 4 blade prop?

[Georgesalmon]

I have a 20' 1972 fibercraft with a 1979 1/2 115hp inline 6 merc. I rebuilt the carbs, fuel pump all new fuel hoses, linc and sync (all with help on this site) and now the engine runs great. Boat was also restored this winter, new stringers, floor, transom, dash's, carpet, gauges, wiring, seats, everything like new now. I added a lot of weight with 4 elect downriggers, 8 - 12 lb balls on board, 9.9 merc prokicker, another battery, 150qt cooler, and fishing gear plus all the normal boat stuff. I had myself and one more on board, will probably fish with three or four. I think the boat originally weighed about 2300lbs, now probably closer to 2800-3000.

The boat came with two props, a SS 19p and an AL 17p. At WOT I get 4800rpm and 36mph with the 19. I'm sure that the PO probably did OK with the 19p prop because of much less weight. I know I need to get down in pitch to bring the rpm up to 5000-5500. I seem to remember that each 2" in pitch changes about 200 rpm. I don't think the 17p will be enough to get me into the manufactures range so I'm not going to waste the gas and try that though I prob should.

So,,,,,,,Any ideas out there? Go to 4 blade, drop to a 15p. Don't know much about 4 blade or how to figure pitch on them compared to three blade.

I'm a Lake Michigan fisherman and will make one run out and one run back usually average 5-18 miles each way. Looking for short bursts of speed and than the trolling motor for several hours.

 

[steelespike]

Re: How far down in pitch? 4 blade prop?

I'm a little surprised at the similarities to the Polar post just above except for the gear ratio.
Did the boat respond well to up trim?Is the speed by gps?
A 19 seems too high for a 1.7 ratio even for the lighter setup. A ss 16" pitch should add 400 to 600 rpm
lower your very high slip,run faster, stay on plane easier.While a 4 blade can be very effective I don't think its needed at this point.
Sticking with ss will make results a little more predictable.

 

[Georgesalmon]

Re: How far down in pitch? 4 blade prop?

The 4800rpm was at max trim of the motor. It seems to be at a good running attitude with the motor in the first trim hole, from there I power trimmed up until I reached 4800rpm and 36mph after that it blew out. It was GPS speed. Actually 36 with the wind and 34 against. Thanks for the info about the 4 blade. Not looking for hole shot if thats what they are good at. There is a prop place a few miles from me "Green Bay Propellor" I know they repair some very damaged props. I'm goning to run down there today and see what I can see. Do you know if a SS prop be re-pitched?

And thank you for the quick reply.

 

[Philster]

Re: How far down in pitch? 4 blade prop?

SS props can be repitched.

 

[Georgesalmon]

Re: How far down in pitch? 4 blade prop?

Thank you, I'm gonna head down there with my prop and see what they can do for me.

 

[Georgesalmon]

Re: How far down in pitch? 4 blade prop?

Well this guy can't repitch it. Nice guy though, could sell me a new one for $325. Wouldn't give me anything on a trade, said to sell it online. Guess I'll try my AL 17p and see where that gets me.

 

[Georgesalmon]

Re: How far down in pitch? 4 blade prop?

Just back from water. The results:
Old aluminum 17p prop with no cupping
WOT 5600 mph 32-33 (gps)trimmed

A little high on the rpm I think. I'm now wondering if a 17p new prop with cupping will lower the rpm, and about how much? I'm somewhat disapointed that I lost speed with a gain in rpm of 700. Must be a LOT of slip with that old non-cupped prop? In normal use I'll have 1 or 2 more people in the boat. I want a prop for all seasons. Try and stay within manufacturer max rpm range. light load and heavy load. To much to ask?

 

[Georgesalmon]

Re: How far down in pitch? 4 blade prop? (NEW AND MORE INFORMATION)

Re: How far down in pitch? 4 blade prop? (NEW AND MORE INFORMATION)

Just came back from the day at the boat landing, changing props and burning gas.

Here is some better information. Kinda starting over to get everything in one post.

I am looking for the best efficiency, lowest possible fuel consumption at planing speed or higher, don't care about hole shot or WOT speed. Fishing only for me, running out and back.

This is what I know:

The boat came with an old merc 19p SS 3 blade prop that gives me 34mph at 4800rpm WOT. Hole shot not so good.

It also came with a 17p AL 3 blade prop that gives me 33mph at 5600rpm, I"m pretty sure it was slipping in the hub so no value except it tells me the motor will run and sounds very good at that rpm.

Today I tried a new omc AL 13.25 x 17p 3 blade prop. I got 36mph @ 4825rpm, max trim w/o ventilation. The best planing speed felt like 23mph @ 3500rpm. Hole shot was OK, no whiplash or anything. Prop ventilated in even a mild turn without trimmimg down

I also tried a new mercury spit-fire AL 13.4 X 15p 4 blade prop. I got 34mph @ 5200rpm max trim. The best planing speed seemed to be 23mph @ 3800rpm. Hole shot really good and it didn't ventilate in a lock to lock turn at WOT.

(all speeds GPS, rpm from new dash gauge)

So that's where I am at. Can't find any other props to try around here and want to do the best I can at hitting it right in one purchase. My finances won't allow a SS prop at this time so it must be aluminum unless I can find a used SS one.

From the above to me it seemed that the 4 blade might be the best choice. Would it stay near 5200rpm with another 400lbs? How much efficiency do I lose at planing speed with 300rpm more? Could I go faster than planing speed and get better efficiency because 2-strokes like to run fast? Is 5200rpm high enough or do I need to get closer to 5500rpm?

 

[steelespike]

Re: How far down in pitch? 4 blade prop? (NEW AND MORE INFORMATION)

Re: How far down in pitch? 4 blade prop? (NEW AND MORE INFORMATION)

Well the 4 blade did just about everything it is supposed to. Slip was best of the lot.
But wot slip seems pretty high. Is the boat unusually heavy or deep or rounded chine
or combination of same. Remind us of the test load?

 

[Georgesalmon]

Re: How far down in pitch? 4 blade prop? (NEW AND MORE INFORMATION)

Re: How far down in pitch? 4 blade prop? (NEW AND MORE INFORMATION)

I think it might be heavier than I thought. My newest calculated guess puts it up at 2800-3000. It's a V-hull, 10" flat in front of motor that goes forward about 4 feet and dissapears. 18 degree deadrise at the transom. The bow entry is a little blunt but that's not in the water. That old engine is rated at the power head I think, not at the prop. If it was a real 115hp I might have a better chance maybe? Or doesn't that matter?