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Old 27 July 2015, 09:49   #1
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minimum engine size for 4.5 rib

hi there just got old 4.5 rib, make is forum favourite Narwhal, and now seeking advice for engine sizing and selection.

The intended use is dinghy sailing support boat. Would like to get the boat to planing but top speed is not important after that.

Currently considering a 25 hp 2-stroke yamaha, would that work? If not then what would be a sensible choice?
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Old 27 July 2015, 10:54   #2
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40 hp at least
What's it rated for?
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Old 27 July 2015, 11:22   #3
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depends on useage,
skiing or multiple persons 40hp+
safety boat or one person 25hp +
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Old 28 July 2015, 10:57   #4
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intended use is safety boat with just one person on board. Will 25hp be enough to get it to plane is really my question. No requirement for top speed just needs to be able to reach downwind mark faster than a sailing dinghy.
the boat seems to have a real V bottom. The CE plate on the transom does not include any HP ratings only max load 1040 kg all up
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Old 28 July 2015, 11:47   #5
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I used to use SR4 with 25 worked fine in similar circumstances,a bit slow to plane due to flooding hull, and on ly ever 1 person in.

I would also say depending on drivers skill lower hp could be better, club boats for instance can be hammered by a huge range of skills and giving a rocket ship over to the less experienced can give issues of there own.

The SR hull weighs in at 350kg do you know the weight of yours for comparison.
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Old 28 July 2015, 14:12   #6
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I'd have said 25hp was a bit light, but will depend on hull weight.

Currently you plan on 1 crew, will that remain? What if a coach wants to go out, or a volunteer offers to crew for you. (I wouldn't consider a 1 manned boat as qualified as a safety boat - can't drive boat and deal with casualty).

What sailing dinghies - 4 fold speed difference between an oppy and a foiling moth...

What is the purpose of the safety boat? Is it to recover bodies? How many? Whats the boat ratio? How far from shore. Always assume the worst happens. Common youth ratio in the UK is 10:1 boats. Lets assume single handed. Can you take all 10 youths on board or would you need to drop ashore and do a second run? If so can you plane with 5 youths plus adult would be part of my question... ...bearing in mind you might be having to make the tricky choice of leaving 5 in the water...

Towing capability? Would you tow 10 boats in a daisy chain back to shore? That doesn't need to be fast but in the conditions that need 10 towed ashore its either windless or windy as ***
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Old 28 July 2015, 16:29   #7
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Quote:
Originally Posted by ShinyShoe View Post
I'd have said 25hp was a bit light, but will depend on hull weight.

Currently you plan on 1 crew, will that remain? What if a coach wants to go out, or a volunteer offers to crew for you. (I wouldn't consider a 1 manned boat as qualified as a safety boat - can't drive boat and deal with casualty).

What sailing dinghies - 4 fold speed difference between an oppy and a foiling moth...

What is the purpose of the safety boat? Is it to recover bodies? How many? Whats the boat ratio? How far from shore. Always assume the worst happens. Common youth ratio in the UK is 10:1 boats. Lets assume single handed. Can you take all 10 youths on board or would you need to drop ashore and do a second run? If so can you plane with 5 youths plus adult would be part of my question... ...bearing in mind you might be having to make the tricky choice of leaving 5 in the water...

Towing capability? Would you tow 10 boats in a daisy chain back to shore? That doesn't need to be fast but in the conditions that need 10 towed ashore its either windless or windy as ***
Some great thoughts there..

I'd like to bet (being an ex-dinghy sailer myself) that one day you'll need to get to someone in trouble fast - seconds can count if someone is pinned under water by rigging or struggling in freezing water in the offseason. I'd go 40>60 personally, never underestimate the benefit of agility..
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