Jerry Montgomery & I were towed by Stan 'Paintboy' in his old M17 (the old one, not the newer one). Stan used a Honda 2HP in the transom cutout. not a breath of wind and calm seas ... speed was about 4kts. M17 = 1800# S17 = 1300# four guys = (at least) 800# -- :: Dave Scobie :: former M15 owner - www.freewebs.com/m15-named-scred :: M17 #375 SWEET PEA - www.m17-375.webs.com On Fri, Sep 25, 2015 at 10:55 AM, John Schinnerer <john@eco-living.net> wrote:
On 09/23/2015 02:16 PM, James Poulakis wrote:
So can a 2hp Honda push a loaded (say 2500#) M-17 at its 5.5 knot hull speed against a 4 knot current? I’m hoping it will because I’m tired of doing the “clean and jerk” with my 60# 4hp Mercury.
It doesn't matter what the current is - hull speed is hull speed. Question is, what is minimum HP needed to push a loaded M17 at hull speed? And, is it worth being able to get the absolute last bit of full hull speed?
AFAIK the curve of power to speed is not linear (not a straight line). The curve gets steeper as you go faster. There is a point of diminishing returns, as you get close to hull speed.
In other words, the last little bit of speed before getting to "hull speed" is going to take proportionally more power to attain than getting "pretty close" to hull speed.
For example, for a boat with a theoretical hull speed of 6 kt., if it takes 1 HP to get to 2 kt. and 2 HP to get to 4 kt., that doesn't mean 3 HP will get you to 6 kt. - it will probably take more than one more HP to get the last .1-.3 kt. of speed.
I experience this personally when paddling - when your body is providing the power directly, you feel the edge between most power-efficient cruising speed and full hull speed pretty clearly. I have to work a bunch harder to get 99% of full hull speed in my kayak than to get 80-90% or so of full hull speed.
So IF (just another example, not based on real world tests) a Honda 2.3 will get an M17 to 5.0 kt. without straining too much, it might still take a 3.5 or 4 HP motor to get to 5.5 kt. without straining too much.
Then it comes down to what is that last .5 kt. (or .2 or .3 or whatever it is in practice) worth in terms of motor cost, weight, fuel use, etc..
Here's a blog I found with a skipper's review of the Tohatsu 6hp SailPro:
https://backbeatsailing.wordpress.com/2013/05/28/tohatsu-6-hp-sail-pro-revie...
From some of the commentary about his choice of this motor instead of the next size up, you can see that he's happy to have a smaller lighter gas sipping motor that will get him "close enough" to hull speed of his boat, rather than the weight and cost and fuel use of 3-4 more HP that it would take to get that last little bit of speed on his hull. And it sounds like this guy is a delivery skipper, in the San Juans, and has done a lot of both sailing and motoring. Also it's a 3000 lb. boat and he's happy with the 6hp...which tells me it's way more than I need for my ~1600 lb. boat.
cheers, John S.