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Old 14 August 2007, 02:57   #1
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yamaha 90 prop

Good morning all

I have an inflatable boat (5meters long, 400 kgs weight) with a 90 yamaha two stroke outboard (model 2004). All together (boat and engine weight around 550 or a bit more kgs). The boatīs deadrise is about 22 degrees.

The current prop is the original 17inch aluminum and the boat is doing in WOT 5300 rpm with the boat loaded with 1 person and 100 liters of fuel (the engineīs recomendation is between 4500-5500 which means Iīm good right?) at this rpm the boats speed is at 35 knts.....with the tank half full and 1 person on board Iīm doing 5400 and 36 knts.

At 3700 rpmīs Iīm getting 22 knts
At 4100 rpmīs Iīm getting 25-26 knts
At 4500 rpmīs Iīm getting 29 knts

I was thinking about a 3 blade 17inch stainless one. Do you have any suggestions?

The goal would be better overall performance (not specifically top end speed.....though I wouldnīt mind better speeds through the rpm range but not in expense of handling because sometimes I come across rough weather conditions).

The boatīs use is mainly for weekend fishing excursions with friends.

I could probably send a photo of the transom with the engine mounted so you could get a better idea.

Thank you in advance for your time.

Moiras Nick
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Old 14 August 2007, 04:06   #2
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Hi Nick,

in an older german boat magazine there was a test of exactly this motor (I have the 85 Enduro version).

They tested the motor on a Sessa sport boat with nearly the same weight like your Rib. The interesting fact on this test was, that they compared the original aluminium prop with a stainless one.

I have to look for the exact size in the evening. But I remember that the stainless prop was not so much better in performance than the aluminium one.
A little better acceleration, a little less fuel consumption and a little more top speed, but not significant. Therefore I decided to keep my aluminium prop in use.
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Old 14 August 2007, 04:26   #3
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Thank you very much idrian. I would be very much interested for the test results.

Iīm not interested in any drastic results....

Iīm working the boat a lot (around 160 hours annually) and the aluminum propellers loose their characteristics quickly. I have two and I change them (and repair the changed one).

Mabe this would change with a stainless.

thanks again for your time
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Old 14 August 2007, 08:29   #4
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I have a rebuilt Yammy 90 (1991) on my 5.4 SR.

Have both 17" and 19" props. Don't have a rev counter (YET!) and with a baby on the way probably not too soon . But with the 17 I get about 34.5 knotts by GPS and with the 19" I get 36.5.

I just use standard Yam Aluminium props as too many rocks around here to hit and if you do so with Aluminium it's a chipped or broken prop, with a stainless prop it's a broken lower gearbox at least.

Ian
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Old 14 August 2007, 13:08   #5
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So Nick,

here are the results of the test. The boat was a Sessa Key Light 18 with a weight of 480 kg (only the boat).

The test says nothing about the pitch of both propellers, only the diameter of 17"

Hope, that helps
Attached Files
File Type: pdf Data sheet Yamaha 90.pdf (11.7 KB, 147 views)
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Old 15 August 2007, 04:30   #6
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Thank you very much Idrian....

the testīs results are really equall between the 2.
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Old 15 August 2007, 05:29   #7
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Hi nmoiras

When i was running the original 17" blade with my set up (which seems similar) i was getting similar figures.

I changed after a lot of thought to a 17" Stainless Titan prop, the prop has much better grip which is better for the routh stuff, a larger surface area on the blades which helps acceleration and i improved my top end performance from 36kts to 39kts but my revs droped to 5000-5100 which is still within yam's recomended.

hope that helps.
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Old 15 August 2007, 05:49   #8
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Hi Ian,

did you notice any chance with regard to fuel consumption with the Titan prop?
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Old 15 August 2007, 06:13   #9
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I've just upped my prop on the K series 85 yam from a 17" standard yam prop to a 19" pitch Michigan Wheel. It's noticeably cheaper to run and my cruising speed is up and feels more comfortable. Max revs have dropped from 5300 2 up to 5000 (max being 5500 on my engine)

It does seem to cavitate a bit easier at higher trim settings though-I'm seriously thinking about dropping the engine down a hole.
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Old 16 August 2007, 18:37   #10
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Quote:
Originally Posted by Idrian View Post
The test says nothing about the pitch of both propellers, only the diameter of 17"
17" diameter sounds a little large. I think the Yam 90 has a smallish gearcase (assuming it's the same gearcase as the F115 - 4.25"?), so a 14" diameter is the largest it'll take.

I would assume the 17" is pitch on the two props. The differences come from different blade shapes and cupping and stiffer/thinner blades and such.

Just as an aside.

jky
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