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Old 15-04-2008, 21:43   #1
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Horsepower / weight ratio

how does the horsepower / weight ratio work please?

My boat is 2.5 tonne. Gear, fuel etc is extra. What HP does it need? We have some strong currents and winds around here....

I want to use the motor for more than mooring / docking. Thanks in advance.
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Old 15-04-2008, 22:34   #2
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Boris,
Horsepower to weight ratio matters in how fast a boat can go but it is not the only factor.

The hull shape is also a major factor in determining maximum speed. Some hulls are designed to plane and some are not. There are also semi-displacement hulls that are a compromise between the two.

For safety reasons, most small boat hulls have a maximum horsepower rating which should never be exceeded. There is usually a placard somewhere on the boat that states the maximum horsepower, maximum weight capacity, maximum persons etc.

What type of hull do you have? Is it a sailboat or a powerboat? What make and length is your boat?

With this information we could probably give you a pretty good idea of the horsepower that you need for your purposes.

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Old 15-04-2008, 22:53   #3
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added info

thanks David.
The boat is a 25 ft fibreglass keeler. Kiwis know the design as a Reactor. 25 overall, 20ft waterline and beam of 8feet. Weight is 2.5 tonne. Keel is 1 tonne. It has a 1gm10 which pushes it along in flat water at up to 5knots. I'm wondering about the options for more power. Engine weight is not a problem as the original Stuart Turner engines were around 130 Kilos. The 1gm10 is around 80Kilos.
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Old 15-04-2008, 23:28   #4
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Boris,
Given what I have seen on other similar sized full displacement sailboats, I would think that a 15 horsepower long shaft outboard would do you fine. You could probably get away with a 9.9 horsepower at the least but this might be a bit sketchy when you need that extra horsepower.

15 horsepower will probably get you pretty close to the theoretical hull speed of your boat which would be around 6.0 knots. It's my opinion that any more horsepower would be an unnecessary cost, unnecessary weight and not really push you much faster after reaching hull speed.

Calculate Hull Speed

Have fun!
David

Edit: I was just looking up Honda outboards and noticed the 9.9, 15 and 20 horsepower are the same weight at 52Kg(116 lbs). If they are close to the same price then what the heck, go with the 20 horsepower.

Honda Marine Outboard Motors - Models Listing
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Old 15-04-2008, 23:50   #5
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You might be propeller diameter limited a bit with the 1gm, but hull speed on that boat isn't likely to be much more than 6 knots anyway. Is the bottom clean, have you cleaned the injectors ever? Both those things will do a lot...

If you increased power a lot (say doubled it), your top speed might only get up to 6.5 knots.
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Old 16-04-2008, 00:43   #6
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thanks for replies.
I have previously read of a factor of 3HP per tonne - someone had said 4HP. Is there a "rule' about this? Is it weight of boat or weight of boat plus crewplus gear etc?
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Old 16-04-2008, 01:30   #7
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Hi Boris. Yes it is all up weight including gear and fuel etc. But the weight does not determine the speed. The weight determines the reaction to the power. If it were a 10Ton boat, but still 20ft, then it would be harder to get the mass moving and harder to stop it. If it were a 100lb boat, it would be easy to get it moving and easy to stop. Neither would go any faster through the water.
There is a rule of thumb that varies. For displacement hulls, it is, minimum of 1.5Hp per ton and up to 3Hp per ton. Above that you are just wasting power. The boat will not go any faster.
The 1gm is probably an ideal match. You are not going to get any more speed than you are now. The theoretical speed of 5.99Kts does not take may other factors into consideration. So 5kts is probably about right and about as much as you will ever get, even if you put in a larger engine. The 1gm is light and that makes a very big difference in your case. You don't want to much weight or you will struggle to sail.
So where are you based???
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Old 16-04-2008, 02:22   #8
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Wind=wellywood, currents=tauranga by the bridge?, I have seen reactors with engines in the 20hp range, the extra weight wouldn't make a lot of difference as they are sturdy sailers, they had pretensions to being a racer but never quite cut the mustard.
With your hp you need to allow for the altenator and compressor if you have one fitted as they will detract from the hp available to the prop. One of the things I would check would be the size of the propellor apperture, if you can't fit a larger dia prop to match any increase in hp then you are stuck ith using an increased pitch which does help but doesn't give the same results as dia.
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Old 16-04-2008, 05:32   #9
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Do these hull speed calculations still work properly if you have 2 waterlines? As in catamaran?


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Boris,
Given what I have seen on other similar sized full displacement sailboats, I would think that a 15 horsepower long shaft outboard would do you fine. You could probably get away with a 9.9 horsepower at the least but this might be a bit sketchy when you need that extra horsepower.

15 horsepower will probably get you pretty close to the theoretical hull speed of your boat which would be around 6.0 knots. It's my opinion that any more horsepower would be an unnecessary cost, unnecessary weight and not really push you much faster after reaching hull speed.

Calculate Hull Speed

Have fun!
David

Edit: I was just looking up Honda outboards and noticed the 9.9, 15 and 20 horsepower are the same weight at 52Kg(116 lbs). If they are close to the same price then what the heck, go with the 20 horsepower.

Honda Marine Outboard Motors - Models Listing
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Old 16-04-2008, 05:54   #10
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See the Malcolm Tennant article, who says multi-hulls are NOT limited by Froude’s Law:
displacement or plane
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Old 16-04-2008, 15:08   #11
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Good read. Thanks, Gord. I keep forgetting I went from like 35' LWL to 27' LWL. I'm expecting this much smaller boat to keep pace with the old one. It's about a knot slower under power and I still haven't had the perfect conditions to see the top speed under sail yet.
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Old 16-04-2008, 18:07   #12
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HP

Boris You may not be getting all the HP avail. What RPM for 5Kts & what is the max RPM you can get? It might be a prop issue.

SSulivann My 2 hulls 36' 2 x 1gm 7.5hp 7Kts @ 2Ltr/Hr same hull with 2x15hp 8kts @4ltr/hr side by side with "sister ship
Regards Bill Goodward
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Old 16-04-2008, 18:17   #13
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You may not be getting all the HP avail. What RPM for 5Kts & what is the max RPM you can get? It might be a prop issue.

Regards Bill Goodward
Thanks for asking, Bill. I don't know enough about powering boats to know why the boat is feeling slow. Here is what I have:

Twin Yanmar 30hp diesels, coupled to Yanmar SD20 saildrives.
I am running at 2500RPM and getting 5.5 knots boat speed.
This setup is aboard a heavy, displacement catamaran with a 27' waterline and a 6 ton displacement.

When you say prop issue, I am with you on that. The boat "feels" like it accellerates very sharply, but then has less top end that I would expect. It feels like the props are "geared down" to me, but I'm no expert on that.

I know the boat had its props replaced at survey, so I'm wondering if they were wrong. They are just the standard aluminum Yanmar saildrive props. Only thing is... I can't recall which diameter he bought when he changed them. I supposed I need to get this boat out of the water a bit and measure (too chilly for a swim). I will update with an accurate prop size as soon as I get the boat up on a mud flat.

For now, do those numbers sound right?
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Old 16-04-2008, 18:56   #14
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Quote:
Originally Posted by ssullivan View Post
Do these hull speed calculations still work properly if you have 2 waterlines? As in catamaran?
Yes, because you still have a bow wake and a stern wake for each hull with a trough roughly midships...which defines hull speed. I'm not saying a full displacement catamaran hull cannot exceed hull speed, they certainly can but that's a different discussion.
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