Max engine size and transome weigh

[Newboy]

The search for an outboard for my new boat is continuing. There's seem to be a shortage of good 75-135hp on affordable price at the moment.

 

Going to look at a Mercury 100 tomorrow, but it is in Ascot and none of the local marine engineers (a handful) do inspections. Managed to find one who does but he insists on engine either on the boat in water or in a test tank (reason given was that engine needs to be properly ran for assessment ). So I ask him to do a basic compression test and a visual inspection and I might take a gamble as long as the block is good, around 135 I've been told.

 

The reason of this post is how does max engine weigh and max hp relate on a boat?

 

The max engine weigh I can understand, if the ob is too heavy it might flip the boat, but does the max hp rule apply if engine is within the max weigh but with too much hp?

 

On my boat I'd been told max hp is 135 but even the importer doesn't know its max engione weigh. Rang a few chandleries and been told most 150 weigh around 180-195kg.

 

The Quicksilver pilothouse 500 which is very much alike my boat but shorter by a foot has max weigh at 180kg but max power at 75. (I have a sneaky suspection that the Beneateau, Octeaue (?) and Jeanneau all uses the same hull as mine).

 

I can see a couple of 150+ engine at 2500-3000 which I know are 180kg. Can I use them provided I don't use full throttle or not? One of the guy even claimed he uses his 200 v6 on a 17 dory which I thought was impossible.

[Afishionado]

I've said it before and no doubt I 'll say it again "Boy ! Do you ask good questions!"

 

Right, physics part 604............ Weight/mass/inertia and all that. The wheight of the engine is distributed across and along the hull and because of physics weighs less in the water than out. DONT ASK WHY IT JUST DOES SEE!!! :lol However its mass stays the same. Now mass affects inertia in the following basic way....The more mass the harder it is to get moving. The more mass the harder it is to stop. So a heavy engine could affect the pick up speed but we are talking minimal amounts in this instance. But if you come to a sudden stop the transfer of mass/ inertia from the engine to the transom could be high and THAT is the feature to worry about. The bozo with 200hp on a 17 ft dory could find 200kg of engine doing 30MPH up his arse if he hit summat going fast.

 

In the world of O/B and boats HP and engine weight don't have much interconnection (unlike motor cars) What does matter is HP and hull shape/displacement.

However it gets very esoteric from this point on and I will get bored trying to remember my physics quicker than you will get bored reading it!!

 

I reckon any modern 75 to 100 will be a really good match with your hull and you would need a stop watch to proove which one went quicker/better. In actual use they will feel almost the same.

 

Mad Mike

[Newboy]

I don't really want anything bigger than 115, but it's getting very hard to find a good 75+. If I don't find anything in the next 2 weeks I might go for a new 70hp Tohatsu, it's a little less than what I would like but at least I would only have to sell my wife.......

[Afishionado]

I don't really want anything bigger than 115, but it's getting very hard to find a good 75+. If I don't find anything in the next 2 weeks I might go for a new 70hp Tohatsu, it's a little less than what I would like but at least I would only have to sell my wife.......

I dare you to show her this post Go on double dare

 

Mad Mike

 

PS After she has killed you can I have your boat please??