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Old 16-07-2012, 09:24   #1
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5hp vs 2.5hp on cal 27...Need your help

I have a 5hp Nissan OB motor on my Cal 2-27. Pushes the boat at 5 knots with dirty hull and about 6 knots with clean hull!

I know this is probably one of those really hard to answer questions.

What would a 2.5hp engine do on the back of my cal? I am not sure if there is any type of formula to figure out speeds and I would probably need thrust information and the website that sells the engine does not have it.

I know that I could always use a 5hp, but wonder if a 2.5hp would push me along.


***side notes. I hate my gas engine. I am looking at lehr engines. I would love to have propane as the primary fuel! The reason for wanting the 2.5 vs the 5hp propane engine is that the runtime per lb of fuel is about double. The only questions is.....Would the smaller engine being 2.5hp push the boat and if so, at what speed?
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Old 16-07-2012, 10:15   #2
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Re: 5hp vs 2.5hp on cal 27...Need your help

The rule of thumb for displacement speeds is 1 HP per 500 lbs. Probably varies a bit with hull form and bottom growth . . . but a good start at least.

So with 2.5 HP you're significantly underpowered. If you're only moving in and out of a slip in calm conditions, with little current, you might be okay. . . . until conditions aren't calm, and then you're in a fix.
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Old 16-07-2012, 10:19   #3
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Re: 5hp vs 2.5hp on cal 27...Need your help

Thanks for the formula. I'm about 6700 pounds. Gives me a 13.4 hp to be "fitted" correct.
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Old 16-07-2012, 10:35   #4
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Re: 5hp vs 2.5hp on cal 27...Need your help

Is there a reason you dislike your gas engine? reliability, smell, gas usage...?
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Old 16-07-2012, 11:12   #5
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Re: 5hp vs 2.5hp on cal 27...Need your help

Quote:
Originally Posted by Cheechako View Post
Is there a reason you dislike your gas engine? reliability, smell, gas usage...?
Good question. Much of the answer is probably just in my head as plenty of people...the majority.....like gas outboards.

history:

When I bought the boat, I specifically looked for one with an ob motor because I don't like the diesel or gas smell inside of a boat. More so the other half as she is much more sensitive than I am. My intention if I bought a boat with an inboard would be to remove the inboard to put on an outboard. The extra storage space can be used bigtime on a 27' boat. I dislike the mess that it can make when work needs to be done among a clean living space with dirty parts too.

I like propane as it is very clean. No spills in the boat, or dink. Very clean. I am fitting out (this week) my instant hot water heater along with propane stove. The fuel will already be on the boat. Going to have stern mounted tanks to keep it outside. The fuel will already be there.

I just don't like the flammable nature of gas along with the smell it gives. I know propane is flamable too, but easier imo to store outside than gas. The minor issue is dependability. I can't speak from experience, but I think a propane engine would be much more dependable than a gas engine when it comes to an outboard.

I know that I could put a 5hp propane engine on the boat for 1600 bucks and a 2.5 hp on for 1k. I know that a 5hp pushes me along pretty darn good right now, but have never tried a 2.5hp engine on the boat. I would be more concerned with how much fuel I burn than speed under engine. If I knew I would get 3.5 knots-4 knots out of a 2.5hp engine, I would go for it. I'm a little apprehensive about buying a 2.5hp for the boat to find out it will not push me at 3.5knots min. Getting double the burn time out of an engine with a loss of 1-1.5 knots would be worth it for my adventures.

I have never tried to put a 2.5 hp on a 27 foot sailboat and wonder how it would work in regards to speed. The idea of a propane engine for 1k is very appealing.

I could always use the small 2.5hp engine for my dink if I did not like it for the boat. And if I did like it for the boat, I could just buy another one for the dink. If I felt that it was underpowered, I could go to the 5hp for he boat next year and just continue to use my 5hp Nissan OB for the remainder of this year.
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Old 16-07-2012, 12:39   #6
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Re: 5hp vs 2.5hp on cal 27...Need your help

This one is a no brainer, in calm wheather the 2.5 will push it but picture you are coming in with a out going tide and a 15 knot head wind? You are moving backwards my friend. I once had a 27 Coranado and I used a yahama 9.9 on her. better to have the power and not need it.
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Old 16-07-2012, 13:21   #7
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Re: 5hp vs 2.5hp on cal 27...Need your help

personal experience. i belong to a sailing club which owns sailboats in the 22 to 30 foot range. we have a mix of outboards on these boats. i personally favor the smallest outboard that will reasonably move the boat, including going out an inlet on an incoming tide. from my experience on these boats, a 2.5 will move any of them in calm conditions. get any kind of a tidal flow and they pretty much stand still. we've found that a 5hp will handle most conditions on the smaller boats but to really punch through the inlet a 9.9 is needed. my recommendation? stick with the 5hp - it's probably the minimum for you.

i also like those propane powered outboards, having had experience with propane powered trucks; the engines ran literally forever. my problem is, will the motor stand up to real life conditions. and this has nothing to do with the fuel and everything to do with the materials used in manufacturing them. so i guess i'm waiting to see some real life experience before making any commitments...

i had an albin vega 27 with a terminally ill inboard engine. i took it out and replaced it with an evinrude 7.5 yachttwin. worked great. and freed up a big chunk of storage space where the engine used to be. and, i could get the engine fixed when it needed it; can't do that with an old swedish made inboard gasoline engine unless you live in sweden....
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Old 16-07-2012, 13:46   #8
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Re: 5hp vs 2.5hp on cal 27...Need your help

Your problem with the 2.5hp is that in addition to having half the power it will also have a tiny prop and a gear ratio designed to propel a 40lb dinghy at 4 knots.

If you could change the gear ratio and a huge great prop it might work. The only solution that comes close is a British Seagull, but if you don't like smells, or own a boiler suit and have a oily rag in your pocket they are not for you. Keep the 5hp.

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Old 16-07-2012, 13:53   #9
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Re: 5hp vs 2.5hp on cal 27...Need your help

I had a British Seagull rated at 3 hp on my Starwind 27 for a couple months. Did fine in calm water, but if there was a headwind or current, I was sitting still at full throttle and fighting to steer myself enough not to drift out of the channel while not losing ground. It was no fun.

Go with the bigger motor.
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Old 16-07-2012, 13:56   #10
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Re: 5hp vs 2.5hp on cal 27...Need your help

wow...as usual, the knowledge on this forum is great and helped me to make a decision....Guys/Gals ****Thank you**for your experience in regards to helping me with this decision. 5hp it is!

The answers on here are exactly what I was looking for!

Again, Thank you.
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