Cat bulkhead design

[montero]

Gonzo do you know production boat with lower unit mounted upside down ?

 

[gonzo]

Yes, it keeps the propellers out of the water so they don't get fouled with weeds.

 

[montero]

I'll send a bottle to anyone who guesses correctly. A bottle of your choice for under $100.

 

[montero]

@fallguy
The price of a 60hp Merc can be quite favorable, even in Europe.
My only doubts were about their "power" and what to do with them if they are too weak.
We just returned from a trip with a 50hp Honda. It had to move a 3.5-ton, 10-meter boat. The steering was a nightmare, despite the hydraulics.
Heavy soap dish with no drive. Brrr...

 

[fallguy]

@fallguy
The price of a 60hp Merc can be quite favorable, even in Europe.
My only doubts were about their "power" and what to do with them if they are too weak.
We just returned from a trip with a 50hp Honda. It had to move a 3.5-ton, 10-meter boat. The steering was a nightmare, despite the hydraulics.
Heavy soap dish with no drive. Brrr...

two 60hp engines will not be a good plan for your SOR; it is simple, but you don’t wish to admit it

 

[gonzo]

My only doubts were about their "power" and what to do with them if they are too weak.
We just returned from a trip with a 50hp Honda. It had to move a 3.5-ton, 10-meter boat. The steering was a nightmare, despite the hydraulics.

To simplify, a 60HP engine has a power of 60HP. The steering problem is not likely related to the power. A 10 meter (33FT) boat should be able to cruise and steer properly with 50HP. Most sailboats on that range have 20-30HP.

 

[montero]

two 60hp engines will not be a good plan for your SOR; it is simple, but you don’t wish to admit it

Why are they so bad? Because of the weight?

 

[montero]

To simplify, a 60HP engine has a power of 60HP. The steering problem is not likely related to the power. A 10 meter (33FT) boat should be able to cruise and steer properly with 50HP. Most sailboats on that range have 20-30HP.

I don't know why it handled so poorly. The maximum allowable engine was 250 hp. Maybe the rear end wasn't weighted enough, which is why it was so difficult to keep straight course.

 

[fallguy]

Why are they so bad? Because of the weight?

You cannot put a 100kg motor and 100kg of fuel on 500kg hull. And for your plan to be ultralight; your hp and speed at 120 will require more significant scantlings, means more weight. Also means more displacement, and so on.

This is where you are having trouble in the design process. You are not actually being rational. A real rational HP max for your plan is an engine you can physically handle alone. This will reduce the hull size and scantling requirements, thus weight, etc.

All the experts left the thread because they don’t like to be bothered.

Just a late edit; the hulls drive the power, not vice versa; generally.

 

[montero]

2 x 60hp is a choice due to a number of factors: available power / weight / mpg. It's rather weak power for given dimensions . I don't want to go full throttle. Big windage is the reason of choice I need some power reserve.

 

[montero]

Most difficult thing for me is make a decision. Because there are many ways to reach the goal and mistakes could be expensive. Now I struggle to keep this 'boat" in one piece on the sea . I cannot say how big are forces to deal with . Mostly intuitive choices.
And its big fun for me.

 

[fallguy]

You need to decide whether the boat will be planing or not. For your wish to be lightweight, displacement is lightest, uses least amount of power, won’t go fast, will handle windage easier (imo).

 

[montero]

For exceptional L/B numbers transition between displacement speeds and planning speeds is nimble. No holeshot.

 

[fallguy]

For exceptional L/B numbers transition between displacement speeds and planning speeds is nimble. No holeshot.

The holeshot is not the issue.

There is just a lot of tension created in 120hp for an 8m cat. The problem isn’t the hp, but the mass at the stern, fuel demand increases, etc.

A displacement boat allows you more room for wine and food and things you may want on a cat on the Black Sea.

And it may avoid the perils of the 120hp design, which are fatter sterns, more fuel requirement, and especially the potential for bad performance. A boat with that much mass aft may hobby horse as bad as mine did (or worse) before we added bow extensions (thanks to Ad Hoc).

 

[montero]

The holeshot is not the issue.

There is just a lot of tension created in 120hp for an 8m cat. The problem isn’t the hp, but the mass at the stern, fuel demand increases, etc.

A displacement boat allows you more room for wine and food and things you may want on a cat on the Black Sea.

And it may avoid the perils of the 120hp design, which are fatter sterns, more fuel requirement, and especially the potential for bad performance. A boat with that much mass aft may hobby horse as bad as mine did (or worse) before we added bow extensions (thanks to Ad Hoc).

The designed length is a minimum of 9m.
There is no chance for any hobby horse.
The weight of these engines is small, IMO.
The forces that will generate max. 600kg each.
Fuel, I am considering integral tanks. I have plenty of space for fuel.