Correct prop?

[dman]

I have a 1999 Angler single engine catamaran with a Yamaha 115 two stroke. I cruise at 20mph at 4500-4700 rpm. WOT is 5400-5500 rpm but I only increase to 23-24 mph. Shouldn't I get more of a bump in speed. The prop came with the boat and is a 3 blade stainless. How do I find out if I am under propped short of buying and trying several different ones?

[Tideline Boats]

Hey Dman, I could use a little more info to really help narrow down the problem. It definately sounds like something is off though.

Let me see if I can help with what is provided though. I will guess the engine is a 1999 saltwater yamaha 115 2 stroke. If so the gear ratio is 2:1, and the WOT rpm range is 4500 to 5500. So you are bumping up against the max wot rating, which is good. (I assume you are not hitting any rev limiters, not sure if they were even on those engines)

The biggest question is what prop are you currently running? (pitch and diameter should be marked somewhere on the prop) With a gear ratio of 2:1 and running at 5500 RPM, even a pretty low pitched prop (lets guess 15" pitch) should be getting you in the neighborhood of 35 mph if you are seeing 10% prop slip. My guess is you are seeing massive amounts of prop slip. First and foremost, you are likely going to want to switch to a 4 blade SS prop. Most single engine cats will perform much better with a 4 blade prop versus a 3 blade. I would also look for a 4 blade that has a decent amount of cupping.

If you can get a good estimate on your boat weight, current prop specs, and some good numbers on current performance; give the guys at propgods.com a call and get their advice. My guess is you are going to want a 4 blade with about 14" or 15" in pitch, but that will greatly depend on comparing to what you are currently running. Hope this helps, good luck.

[davidkuhlmann]

In addition to what Tideline said what length boat is this and what is MAX HP on it? Sounds to me like you are propped correctly but might be underpowered. I second Ken at www.propgods.com

Edited August 16, 2011 by David Kuhlmann

[dman]

Thanks for the replies. I will have to look at the prop for the numbers. It is a 19 foot that I believe weighs 1900 lbs. I hold 75 gallons of fuel so that is another 600 lbs. I guess a couple of hundred more with gear etc. It is rated for a 150 hp max.

[Andya]

I would agree that you should see more speed out of the boat. My guess is you may have a mounting height issue, incorrect prop, and or you are carrying more weight in the boat than you think. (something holding water...)

[davidkuhlmann]

Yes you are a tad under powered for the weight that you are hauling. But I think you have excessive prop slip. You may gain a bit more top end speed with a quality SS prop. Again check with Ken at Prop Gods. Your RPMS are good but that prop might be able to have some cup added to give it more bite as well. I'd go talk to a local prop shop in your area and see what they say. Make sure you tell them what you posted here above as well.

[dman]

I checked the prop and the only thing on it is "17-K"

[davidkuhlmann]

Did you contact Ken at Prop Gods?