Need some prop help advice please. Yamaha 90 stroke

Pale Rider

Seaman Apprentice
Joined
Jan 6, 2014
Messages
47
ok, had a white oem 13x17K prop on the yamaha 90hp 2 stroke and was seeing 51-5300rpm no problem...

had an incident with the only object during low tide and banged up the prop, had a cresent wrench to straighten out the bend so we could get in.
Pulled the boat and ordered a new Sols 13x17 Amita3 prop from iboats.
Installed the prop and went for a splash and now I`m down 500-600 rpm, only turning 4500 and the engine feels loaded down and not so easy to get on plain...

otherwise cruising at 4000 rpm is ok...

what did I do wrong?

speaking with live chat when ordering to be sure it was the right prop for the application and indeed the oem was a 13x17... confused to say the least. what can I do? Thanks
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
You just experienced one of the issues many folks run into when changing from one prop manufacturer to another. Props can be designed with many performance characteristics in mind. Some for top speed at the expense of hole shot. Hole shot at the expense of top speed. anything in between is called compromise. Blade shape, rake, and cupping (or lack of it) all have a bearing on how a prop performs. This is why prop selection is such a crap-shoot. It is always best to stick with the same manufacturer if you are happy with that props performance. If you switch manufacturers, it is best to work with a supplier that will let you test them until you find one that meets your needs. Actually, your original prop sounds a little to much pitch if 5100 - 5300 is all it would deliver. Seems to me that engine should spin up to somewhere between 5500 - 5800 rpm. Point being the engine was already "lugging" with the original prop.
 

robert graham

Admiral
Joined
Apr 16, 2009
Messages
6,908
Maybe get your old damaged prop rebuilt/repaired...keep your new prop for an emergency spare?...
 

Pale Rider

Seaman Apprentice
Joined
Jan 6, 2014
Messages
47
You just experienced one of the issues many folks run into when changing from one prop manufacturer to another. Props can be designed with many performance characteristics in mind. Some for top speed at the expense of hole shot. Hole shot at the expense of top speed. anything in between is called compromise. Blade shape, rake, and cupping (or lack of it) all have a bearing on how a prop performs. This is why prop selection is such a crap-shoot. It is always best to stick with the same manufacturer if you are happy with that props performance. If you switch manufacturers, it is best to work with a supplier that will let you test them until you find one that meets your needs. Actually, your original prop sounds a little to much pitch if 5100 - 5300 is all it would deliver. Seems to me that engine should spin up to somewhere between 5500 - 5800 rpm. Point being the engine was already "lugging" with the original prop.

a little back ground:

bought the boat used, did a sea trial and the boat ran well but only turned 51-5200 at the time. we tested so we could see if the engine had any fueling issues. The paid mech checked compression, was found at 120 on each hole and he said the engine was fine, and the boat was a nice buy.

after I cleaned it up and re rigged the boat and after bottom paint, (boat had no bottom paint on it when purchased) I also cleaned and repainted the lower case and the prop. we splashed her when we were ready. within a few minutes at the ramp while pulling away from the dock we felt a kick and it was obvious we hit a submerged object or rock. after getting into deeper water, I raised the outboard and saw a bend on the trailing edge of the prop, and straightened it out and continued to our destination.The boat ran the same and maxxed at 52-5300 running at a good clip for about a mile then cruising speed the rest of the way.

since this prop had seen better days it was a good idea to just buy a new one and turn the other into a wall ornament. so I looked up the prop chart on iboats and chose the same pitch... now it is night and day...

any recommendations for a starting point with a lower pitch? 13 x ?
 

Pale Rider

Seaman Apprentice
Joined
Jan 6, 2014
Messages
47

Pale Rider

Seaman Apprentice
Joined
Jan 6, 2014
Messages
47
just got back from a water test with some numbers

stats:
2 adults = 330 #`s combined
2nd group 24 battery in CC.
Full tank of fuel minus approx 4 gals.
gear/tool bag.
bimini top up.

conditions:
incoming tide in lagoon/channel/river
wind approx 10-12 mph
water - light/meduium chop

idle speed in lagoon/channel to river: 800-850 rpm`s - 3.6 mph

underway:
1500 rpm - 5.8 mph
2000 rpm - 7.25
3000 rpm - 19 mph
3500 rpm - 25 mph
4000 rpm - 29.5 mph
4500 rpm - 33 mph
5100 rpm - 35.7 mph max rpm, max speed.

all speed from Lowrance GPS.

boat can get up on plane decently and cruises nice between 3500 - 4000 rpms.

boat feels heavy and somewhat struggles to get past 4500 rpms and really need to trim the boat out more than normal to reach 5100 rpms... engine def feels loaded down and throttle is pegged wot to get max rpm and speed.

is this as good as it gets?
user_offline.gif
 

Pale Rider

Seaman Apprentice
Joined
Jan 6, 2014
Messages
47
only water in bilge is from the rain. foam in area`s I`m able to see is dry.

this is what I`m told:

engine is to low even though the AV plate is 1" above the keel/bottom of boat. The height of the engine has not been changed since 2002 when new.
the speed numbers are on par with other like boats with same scenario, but I feel the 90 is working hard to get past 4500 with the boat trimmed out enough to cause the hull to porpoise.

I`m pretty sure the PO left the engine in it`s unchanged mountings and ran the Yamaha prop. The boat was rigged to fish so I don`t think max speed was a concern for the PO... just guessing here.

But I`d like to make it easier for the boat to plane out while loaded with fuel, passengers and gear, and easier to cruise with a new prop choice. This current solas prop requires every bit of umpf this little 90 has. somethings has got to make this run a bit better...
 

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