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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 02-17-2002
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motor size and speed

I know sailing isn''t about the motor but.....
if manufacturer recommendations call for a max of 10 can I put a 12hp or even 25hp?? If so does my boat truely go faster or am I losing so much maneuvarability that I am actually losing speed? Planning on lots of ICW travel and woud like some informed opinin or insight. Thanks in advance Heather
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Old 02-17-2002
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motor size and speed

Its like this, displacement boats have a hullspeed (generally around 1.34 times the square root of its waterline). It takes relatively little horuse power to achieve that speed (roughly 2 hp per long ton) in flat water and no wind. As much as 50% more hp can be used to stay at hullspeed in waves and adverse wind conditions. But beyond that more HP will cause the stern to settle and begin to actually slow the boat down. You can add a lot of hp to even make small gains at that point but you''ll burn a lot of fuel, your engine won''t like it and you really won''t get anywhere noticeably faster. (All of this assumes a non- planing hull.)

When a manufacturer posts a max hp power number that is generally the maximum HP that will do your boat any good and it is usually the maximum hp that your boat is engineered to safely withstand. If it says, 10 hp max, I''d stay at or below 10 hp max.

Jeff
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Old 02-18-2002
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motor size and speed

I have Niagara 26 with 1700 lbs ballast,4000 lbs displacement, 26''8" LOA, 23''0" LWL, 8''4" beam. Currently using Honda 7.5 4 stroke circa 1979. The motor works reasonably well but is due for replacement.

I have been considering long shaft 9.9 but recently saw a Yamaha 8HP high displacement O/B which had a much larger and flatter prop. When in choppy waves attempting to motor thru 35+ Knots wind we were making approx 2 knots speed over ground. In this situation would the 8HP high displacement be the way to go or should I condsider 9.9 or more? Weight has something to do with it but I am more concerned about having lots of motor when I get stuck in bad conditions.

Any thoughts or comments?

Thanks,

Mike Hoyt
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Old 02-18-2002
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motor size and speed

If you can maintain 2 kts with that engine in those conditions, I think you will be very pleasantly suprised with the performance of a new engine of the same aproximate hp. As Jeff related, more hp will not push your vessel faster than its hull speed. It will only push it up on the bow wave and down on the stern wave causing it to "squat"
Your comment on a long shaft is important. I see lots of auxs coming out of the water in chop. If you are concerned with weight, be sure to check the specs on any 4-stroke you consider. Weight in these engines is becoming an issue.
In any vessel the SOG relative and important BUT,with the worst case it becomes possible only to hold the bow into the conditions. To me this is the critical situation, and the real need to answer with an engine in extreme conditions. Just a few thoughts.
JEF
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Old 02-18-2002
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motor size and speed

Please elaborate on "pleasantly surprised". In my previous post I meant to day High Thrust for the Yamaha.

In that day of bad judgement we were able to keep the bow to the wind ... barely. There was a lot of banging each time we came down off a wave. Did not like it at all!

Does your comment indicate that more HP or thrust would or would not be good in those conditions? I would definitely have preferred an inboard at that time but that is not an option.

Some notes.
On the occasion mentioned the prop never came out of the water.
The engine housing sometimes was partially immersed.
The bow wanted to be blown around but we were just able to maintain steerage.
After particularly large waves esp. when two in a row our 2kt. speed often dropped to near zero.

I am looking to replace the old engine although it moves us along at 5.5 - 6kts in most condidtions. In a high winds and waves it is not so good. What is best type of Outboard for my vessel and my state of mind?

Thanks,

Mike
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Old 02-19-2002
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motor size and speed

Mike,
Sounds like you had quite a ride!!
If your bow was slapping, you probably did not want to go any faster. That would have made for an even rougher ride.
I take it that your outboard is mounted on the transom. With this arrangement none of the thrust coming off of the prop is helping you maneuver. With a prop in front of the rudder you get more maneuvering ability because you are vectoring the thrust.
You have experienced the reason my S/Y was modified by adding an engine well in tbe cockpit. This places the prop in front of the rudder and I enjoy very good low speed maneuverability.
Your question of HP in newer engines. They are rated at the prop now and therefore a 10hp new may preform as an older 8hp. I believe (help here Jeff) the 79 was rated at the output shaft.
At any rate the new engine will preform much better than the old tired one.
The good news is that you have found the operating limit of your vessel.
Congratulations. It is very important to understand that each vesel has performance limits. Knowing them will keep you out of situations that the vessel cannot handle.
I have had to hold position on my F/V because of wind and wave conditions. I simply could not make headway. So I''ve waited it out and /or taken a safer route to an alternative port. It''s all part of the wonderful world out there.
Safe Sailing.
JEF
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Old 02-19-2002
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motor size and speed

JEF,

Sorry about typos in earlier posts.
Outboard is mounted on cutout in transom. Manufacturer claims this gets the prop out of the cavitation that it is normally in when on a bracket.

The motors are not cheap but are very necessary in some conditions.

Where I sail I am in a narrow Starit (Northumberland Strait between NS and PEI) and it is quite shallow. In our trip last year we ended up motoring straight into the wind to make our Bay while crossing the strait.

I too am very interested in Jeff H input since he is very knowledgable in these areas.

Thanks again,

Mike Hoyt
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Old 02-21-2002
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motor size and speed

I am also very interested to hear what Jeff_H and other very experienced sailors have to say. Should 25 foot boats go for 9.9hp motors? 8HP High Thrust as indicated any better? How about 15HP? Less HP, more HP, High Thrust? Traditional?

I don''t know! I have a boat that came with a motor. I am also considering replacement. What is best for a C&C25 Mk I? Will any of these make any difference in choppy seas?

PLEASE HELP!
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