Repower challenges

[jim rosenthal]

That sounds a bit small to me...I can imagine that >20" diameter might not fit under there, but 17" pf pitch esp on 3 blade wheels doesn't seem like enough.

[Captain Ralph]

Taking 4 inches of pitch out should have picked up a lot more than 300 rpm. With a 1.5 to 1 gear you’re just churning water. That’s a wide heavy boat that you’re trying to push and it takes diameter to get that hull on top. You need a pair of 1.75 to one or 2 to one gears to turn the size wheels it takes to get that boat on top.

I agree, without reduction, more diameter, maybe less pitch. 4 inches out may have been much.
I wonder if he is under pitched and the governor has stopped at max RPM?

I hope we have not chased the OP away.

[dsharp]

It does sound like he hitting the governor. I was looking on the net since the Yanmars got popular after I got out of the business full time. The Yanmar site had a 36’ Azimut that had the same engines with 2 to 1 gears and it was making 26 knots. It did appear to be about 1000 pounds lighter. I wish they would have listed the wheel size

[Pascal]

It does sound like he hitting the governor. I was looking on the net since the Yanmars got popular after I got out of the business full time. The Yanmar site had a 36’ Azimut that had the same engines with 2 to 1 gears and it was making 26 knots. It did appear to be about 1000 pounds lighter. I wish they would have listed the wheel size

1000 lbs lighter? I don’t buy that. Mutts are built brittle light.

[cww]

I agree, without reduction, more diameter, maybe less pitch. 4 inches out may have been much.
I wonder if he is under pitched and the governor has stopped at max RPM?

I hope we have not chased the OP away.

He needs less diameter and more pitch with those shallow reduction gears and high rpm engines. What he really needs is a new prop shop, you can't just replace 2800rpm engines with 3300rpm engines and use the same gears and wheels, they need to be smaller but may not need less pitch.

Pitch is what actually moves the boat forward, diameter is just how much blade real estate you distribute the load over to achieve a reasonable level of cavitation. It's not a false dilemma between diameter and not, if he needs more blade area just go to 4 blades it's not a big deal. Diameter is limited by the rpm range of the engines and the depth of the reduction gears because the blade tips spin faster than the shaft and you get adverse affects like disruption of flow and exponentially higher drag if the blade tip speed exceeds usually somewhere around 50M/s on a normal boat propeller or mach 1 on a plane prop. It looks like exactly what he's describing, burning a ton of fuel and going nowhere slowly.

He has 2 options, the expensive one is go to 2:1 gears to save money on props, which is obviously silly. The second one is order new props with a competent shop doing the calculations. I would bet $1,000 they wind up being less diameter.

Here's a quick and dirty explanation: MP-Aug-Sep14.pdf (hydrocompinc.com)

[RandyRampage]

Did you have a good prop shop run the numbers for the new engines? When I repowered my 53, the calculations were spot on. Are you sure the shafts can handle the new engines torque?At 13 knots the boat isn’t on plane so yes you ll be burning ridiculous amount of fuel as soon as you get above hull speed which is likely around 7.5kts for a 36. What’s the HP rating on these engines? Have you checked the specs on exhaust requirements? I had to upsize the exhaust for my 6CTAs. Trim tabs working ? I’d start by making sure a prop shop runs the number and make sure the gear ratio etc is correct

The smoke was brown before the rematch, now its white maybe a little blue. The H.P RATING is 330 metric h.p. The boat does not have trim tabs but they are on my list. Thanks for your reply