HP rating for a 1970's boat

[Nos4r2]

My Microplus 500 is rated up to 85hp, but she was built in 1976 before outboards were rated at the prop. She only has a 15" transom (see pic of an identical boat).

<br /> The maker went out of business in the 80's so I can't ask them this either, BUT...<br /><br />1)If I put a recent 85hp will she be overpowered or is this rating likely to be due to weight because of the short transom-in which case<br /><br />2) Can I put a 90hp merc 2stroke from around 88 on her as the 70hp and 90hp are all based on the same motor and should weigh the same.<br /><br />Common sense tells me not to do it, but there are always exceptions, depending on the variables the manufacturer used to arrive at the rating.<br />I'm bearing in mind that anything above 85hp in '76 was a darn sight heavier lump than it is today-the merc 90's were IL6's and the OMC's were V4's and physically a lot bigger and heavier than a 3cyl merc 90.<br /><br />This is only a thought as I'm broke at the moment but I'd like more power!<br /><br />Opinions here are valued. Cheers guys.

 

[bernieb]

Re: HP rating for a 1970's boat

A really great looking boat style. The boats of those years were built really tough and no doubt a lot of extra weight in the hull.If I were you I'd go to a reputable dealer and get his opinion and look at new boats with horsepower already mounted to give you an idea.Weigh the boat before you go.You might get on Yamahas web site and go to the performance to check new boat weight and style and hp ratings.Being broke and having a boat fever ain't all that bad,it'll sure get you thinking on how to make money.

 

[JB]

Re: HP rating for a 1970's boat

I think you would be okay with a modern 85 or 90, Nos4r2. I would go with the JohnnyRude V4 for the added cubic inches and low end torque, though. I never have liked the way the Merc triples idle or troll.<br /><br />Your Microplus sure is a nice looking rig. Looks like it would be a fairly simple matter to bring the transom up to 20".<br /><br />Good luck.

 

[WillyBWright]

Re: HP rating for a 1970's boat

You may bwe able to get the boat re-rated based on modern formulas. I have no idea how you'd go about that in the UK. But I've heard of it being done here in the States.

 

[Nos4r2]

Re: HP rating for a 1970's boat

She's 1100lb(ish) unladen according to the original specs with maybe 200lb of gear on top but with the cabin and empty cockpit a lot of that is forward.<br /><br /> Having looked at the Yamaha site I think maybe I'm being unrealistic in wanting to go above 85hp-the 90hp rated bass boats on there are a good 500lb heavier-and she IS only 16'6" long after all.<br /><br />Unfortunately there's no realistic way to raise the transom height without major work as the reinforced area in the transom is extremely strong but not very large(about a foot bigger than the clamp and level with the top of the white line in the picture)-I suspect the extra weight will cause the transom to crack if I put a longshaft on her so a shortshaft is the only option. It's a bit of a pain as shortshafts aren't that easy to come by in the larger sizes.<br /><br />If I do put an 85 on her smoothness at trolling speeds isn't really an issue anyway as I've got a 5hp aircooled kicker with a charging circuit for that purpose. Saves a fair bit of fuel too.<br /><br /> Thanks for the comment JB!