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Catalina/Capri 25/250 Sailor's Forums

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Outboard "boat-pushing power"

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RichardG
Admiral

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USA
990 Posts

Initially Posted - 12/06/2002 : 12:38:46

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>I'll bet yours will pull the other boat backwards.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote> -- Dave Bristle (from another thread)

This would mean a 4-stoke is more efficient at converting horsepower to "boat-pushing power" ("bpp" -- for lack of a better term). While this may sound off-the-wall, in order to be able to compare apples to apples, is there a better quantitative measure of true bpp than horsepower when comparing two engines, regardless of whether they're a 2-stroke or a 4-stroke or the specified horsepower? For example, could the bpp of a 9.9 hp 2-stroke be equivalent to the bpp of 6 hp 4-stroke?

Plus, I suspect there may be differences in bpp between engines of the same hp, but different years (e.g. 1990 Honda vs. 1999 Honda 8 hp, both 4-stroke). Also possibly different engine makers, props, etc.?

If so, a Tug-of-War tournament may be a good assignment for Practical Sailor.

Also, does a Tug-of-War contest test only "starting power"? How would you test "cruising power" (or is there any difference between the two)?

RichardG 81 C25 SR/FK "Sanity"

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Douglas
Master Marine Consultant

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1595 Posts

Response Posted - 12/06/2002 : 15:27:20

You guy's have way to much time on your hands.

Doug&Ruth
Triska (Alberg 29)
Tacoma Wa.

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Ed Montague
Captain

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USA
499 Posts

Response Posted - 12/06/2002 : 15:54:33

Hi Richard, I think that the telling difference in the horse power debate isn't in motor head, it is down at the prop. The newer 4 strokes have the same rated HP as the older 2 cycles but the newer high thrust props make all the difference. My old 15hp 2 cycle Evinrude doesn't come close to the backing power of my new 9.9hp Yamaha with a larger diameter prop. Unfortunately, the cavitation plate to prop center line distance will limit putting these larger props onto an older motor. The transom on my boat was not designed for all the torque and causes some concern and caution.

Ed Montague on 'Yahoo'
1978 #765 SK, Stnd, Dinette ~_/)~

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 12/06/2002 : 16:35:20

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
The transom on my boat was not designed for all the torque and causes some concern and caution.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
Ed: I think you're right to be cautious... When I put a new bracket on for our new 108# high-thrust, XLS Honda, I put a fairly large Starboard plate on the outside of the transom, and two smaller ones inside (above the shelf) to spread the load. I used two inside, rounded at the corners, because the fiberglass is more curved in there--I wanted them flat on the glass, not pressing at the corners. You can work Starboard with standard woodworking tools--I used a circular saw, orbital sander, and electric drill.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 12/06/2002 : 16:45:53

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
...the newer high thrust props make all the difference.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
I agree, and there's another innovation from the Japanese: Honda and Yamaha vent the exhaust so it comes out through the prop hub in forward gear, but foward of the prop in reverse. That way the bubbles don't get to the prop blades where they turn to foam and greatly decrease the bite in reverse.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

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Brooke Willson
Admiral

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USA
983 Posts

Response Posted - 12/06/2002 : 17:23:08

I've been shopping outboards, and Nissan/Tohatsu makes a 9.8 2-stroke using 100:1 gas:oil mix, and with the 25" shaft weighs 68 pounds.

The engine weight is a lot more important to me than torque. I mean, hull speed is hull speed, whether it's at half or full throttle.

But I don't sail in California, either.

Brooke

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Arlyn Stewart
Master Marine Consultant

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USA
2984 Posts

Response Posted - 12/06/2002 : 19:47:10

Dave, regardless of the literature....I'm not sure the new Hondas exhaust out the prop either direction....at least not in my trash can.

And, the 90's generation Honda 8 had a lot better reverse thrust when the restricter was snapped off the shift linkage allowing more than an idle in reverse.

<img src="http://www.cox-internet.com/arlynstewart/images/helming.jpg" border=0>
Arlyn C-250 W/B #224
R&R N/E Texas and Great Lakes

[url="http://www.cox-internet.com/arlynstewart/"]Arlyn's C250 Mods n Cruisin Stories[/url]

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 12/06/2002 : 21:31:16

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
Dave, regardless of the literature....I'm not sure the new Hondas exhaust out the prop either direction....at least not in my trash can.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
I can't argue that--haven't run mine in a trash can. But apparently they're keeping the exhaust away from the prop.
<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
And, the 90's generation Honda 8 had a lot better reverse thrust when the restricter was snapped off the shift linkage allowing more than an idle in reverse.
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
My '95 throttled up pretty well in reverse, but didn't pull half as well as my '02. I doubt that the PO snapped anything off--he knew very little about boats and motors.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette-Honda "Passage" in SW CT

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