yamaha 90 prop

nmoiras · 14 August 2007, 06:57

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 boat´s 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 engine´s recomendation is between 4500-5500 which means I´m good right?) at this rpm the boats speed is at 35 knts.....with the tank half full and 1 person on board I´m doing 5400 and 36 knts.

At 3700 rpm´s I´m getting 22 knts
At 4100 rpm´s I´m getting 25-26 knts
At 4500 rpm´s I´m 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 wouldn´t mind better speeds through the rpm range but not in expense of handling because sometimes I come across rough weather conditions).

The boat´s 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

Idrian · 14 August 2007, 08:06

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.

nmoiras · 14 August 2007, 08:26

Thank you very much idrian. I would be very much interested for the test results.

I´m not interested in any drastic results....

I´m 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

walruz · 14 August 2007, 12:29

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

Idrian · 14 August 2007, 17:08

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

nmoiras · 15 August 2007, 08:30

Thank you very much Idrian....

the test´s results are really equall between the 2.

ian mcconnell · 15 August 2007, 09:29

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.

Idrian · 15 August 2007, 09:49

Hi Ian,

did you notice any chance with regard to fuel consumption with the Titan prop?

Nos4r2 · 15 August 2007, 10:13

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.

jyasaki · 16 August 2007, 22:37

Quote:

Originally Posted by Idrian

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

Idrian · 17 August 2007, 07:57

You're right. My fault. 17" inch diameter is definitely a little to large :-)

ian mcconnell · 20 August 2007, 12:59

NOS4r2 mine cavitates rather too easily as well, i have a weight balance issue on mine.

are you on the lowest hole already, or do you intend to take a chunk out of top of the transom to lower outboard???
keep us posted it seems to be an issue on the searider!

Nos4r2 · 20 August 2007, 15:59

Quote:

Originally Posted by ian mcconnell

NOS4r2 mine cavitates rather too easily as well, i have a weight balance issue on mine.

are you on the lowest hole already, or do you intend to take a chunk out of top of the transom to lower outboard???
keep us posted it seems to be an issue on the searider!

I'm on one hole up from the lowest. I think I should be Ok once I drop it that extra bit.

walruz · 20 August 2007, 17:12

I run mine on the lowest setting. I would NOT take anything off the transom.

No slippage or cavitation problems, BUT, I probably get a lower top speed. Also my hull is flooding while I believe (and stand ready to be corrected) that you have yours plugged up NOS.

Ian

Nos4r2 · 20 August 2007, 17:24

Quote:

Originally Posted by walruz

I run mine on the lowest setting. I would NOT take anything off the transom.

No slippage or cavitation problems, BUT, I probably get a lower top speed. Also my hull is flooding while I believe (and stand ready to be corrected) that you have yours plugged up NOS.

Ian

Mine is sealed. I'm only on the hole position I'm on because that's where it was when I bought it. My cav plate is about 5mm above the bottom of the transom.

walruz · 20 August 2007, 18:10

Thing is it only makes a difference on the hole shot as once on the plane it doesn't matter. I haven't measured where I am against the keel as Freedom won't see her trailer until at least October

.

Ian

Nos4r2 · 20 August 2007, 20:53

Quote:

Originally Posted by walruz

Thing is it only makes a difference on the hole shot as once on the plane it doesn't matter. I haven't measured where I am against the keel as Freedom won't see her trailer until at least October

.

Ian

Yep. I think I prefer the sealed hull in rough weather though-holeshot in bad seas can be quite important which is why I set up the flap on the flooding hull on my old sr4.