Too Underpowered..??

[finding_time]

I'm with Kerry on this one the boat can take a 90 but will it be a better boat? There seems to be a growing obsession with more power these days and i reckon it killing alot of boat performance( i'm not talking about top speed)

My southwind 5.2 longboat had a 80hp motor on it when i brought it, but if i ever repowered it ,it was getting a 60. I know bill corten had at one time a 225 on reel affair but has dropped back to a 175( i think) and his boat lives in the south passage bar( anyone that tell you extra hp over normal power in a bar helps is a fool and has done little bar work)

I had the use of a 6m seafarer for years it had a 200hdpi on the back and went like crazy (100km per hr on the gps) i since spent time on another 6m seafarer it's got a 150 on the back and is a much better boat but does not have the top speed ( how ofter can you go 100km/perhr offshore)

Imho it's all about balance and if you put to big a donk on a boat it's weight in a bad spot and you will compromise other feature of your vessel. just think of fishing rods , your always after a balanced set-up no good putting a 6500 sized reel on you 1-2kg bream outfit

Ian

[shags101]

if the 75 an the 90 are the same engine same wieght same displacment but the 90 revs higher to get the 15 hp the preformance will be the same except the 90 will go a tad faster (how fast do you want to go) use more fuel not any real advantage unless you are going racing or want to impress people by quoting i got a 90 hp on the back save your money go the 75

[jackal]

I have a 4.8m quintrex bowrider with a 60hp 4 stroke yamaha. I thought it was too slow for first so I put a 13pitch prop on it, heaps better sits on 20knots at 4000 revs, I use the 13 pitch prop on calm days such as bays and if a bit rough I go back to the 12 pitch. I would say the 75 hp would be ample unless you want to ski.
regards Jackal

[bastard]

Id put the maximum recomended horsepower on it.

[WHALEOIL]

I am running a 90 yamaha 2 stroke on a 6.3 metre Aluminium. That is heaps of power for me.

[Bowser]

The only real way you can decide this is by testing the hull with the motors on it under normal operating conditions. Different hull brands perform differently under differing conditions of load and sea. e.g. I note that most Quintrex boats have a lot more horses on the back then similar length boats. Now this is either that they are all a bunch of power nuts or the hull needs this extra power. To my mind this has a lot to do with the flare of the bow. This seems to lead is a much reduced planing surface and consequently the need for more grunt to get on the plane and to stay there. I don't know what brand of hull you are considering (you have only stated hull weight) but do know that each hull has a speed that is comfortable in most conditions regrdless of the horse power and unless you are really going to load the hull up e.g. with 6 dive tanks, ancilliary gear etc then fitting maximum recomended horsepower is overkill and a waste of money. All motors have a most economical rev band and this is where you want the motor running most of the time. A larger motor won't neccessarily mean increased economy because it is supposedly running at lower revs. You may find that the most comfortable cruising speed is outside your power band and you end up with more surge and loss of power caused by the conditions and the motors lack of grunt and end up with higher fuel use. A larger motor and consequent larger prop may affect your ability to effectively slow troll.

The right motor is the result of large variety of variables such as planing length, normal weather conditions and payload, expected usage, effect of the weight of the motor etc etc, and can only be assessed from trial and error on your hull. Unfortunately in the boating world this is bloody hard to do unless you are buying the boat and hull from a very large retailer who has the same model sitting around with different motors on them and is prepared to let you test drive each of them.