MPH per 1000 RPM

[rfoll]

I have a 20' Ski Centurion Barefoot Warrior with a fuel injected 175 HP Mercury outboard on it. I used my GPS for speed and ran a series of tests to find my best fuel economy speed. The results are not what I expected. I started at 28 MPH and ran up to 45 mph, recording RPM at each 3 MPH increase. At 28, I came up with 8.75 mph/1000 rpm, and it went all the way to 10.38 mph/1000 rpm at 40. Top speed at 55 mph showed 10.58 mph/1000. At that speed the boat is becoming a flying beast that requires your full attention, now way is economy very good up there. I expected to see Prop slippage at higher speeds, possibly as a result of increased drag from both the air and water, indicating by diminishing speed for added rpm. This boat doesn't really rise high out of the water like the bass boats, because it is designed to hold on tight to the water with a skier in tow. Anyone have any thoughts on this?

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[Kevin Johnson]

I read about this just briefly but that indicated there are numerous variables that will affect your "mileage". The production of the boat has ended but there are many of them out there and other owners could be asked about their results, keeping in mind that different engines, props, water conditions, prevailing wind, trim, hull condition, weight, etc. can all affect your results. You are definitely doing the correct thing by collecting data. Wish I could offer more help.

[jake197000]

trim tabs can do wonders.i wouldnt have another boat without them.

[MadBill]

Your results show that the effective "gearing" improves with speed, which may just mean the prop is well suited to the combination. Fuel consumption though increases with engine speed and fuel efficiency is best at WOT, so I don't believe you can arrive at any worthwhile conclusions without some way of measuring fuel flow.

[rfoll]

I am starting to think it is related to prop rpm, with the prop becoming more efficient higher rpm. The reduction in the lower unit is 1.85, nearly 1/2 of engine rpm. The High Five prop was originally designed to be a surface runner for high speeds in the bass boat world, but eventually lost out to other designs. It is a very effective anti-cavitation prop, somewhat of an advantage for my hull design. It is also tough as nails, the shorter blades don't bend or break. It's possible if I had a different design it might show different results. With good stainless props costing $300 and up, buying them to experiment is not cost effective If I were after a fuel savings.

[ParDeus]

You might laugh, but I had a similar boat, 200hp Merc, and bought a hard rubber/plastic-y prop for those unexpected Florida sandbars.

The boat pulled great from 0-20ish mph, then went very inefficient from 20-30, and above 30mph my fuel consumption was cut by at least 1/3rd.

YMMV. (Finally get to use that, lol)