Motor well size for dual 60s or 75s?

[erikgreen]

Hey guys -

I'm working on building a new boat (just got started, I'll put a thread in the building and restoration section soon) and I have a design question related to dual outboards.

The plans I have for the boat include a design for a single outboard motor well, and I want to go dual instead, or at least leave space for duals in the future. I'm not going to exceed the rated power for the boat, just divide it in half and spread it among two outboards.

I know the ABYC folks have standards for motor well size, but I don't have a copy, and they cost money to get. Basically, I need to know how much transom space dual 60s or maybe 75s would need and how much motor well depth and width. I could just build a full width (8 or so foot) transom and well, but I'd like it a little narrower than that if possible.

I found in new motor specs that the standard is mounting on 26 inch centers, which says to me 2 motors would need 78 inches minimum width, or 6 and a half feet, but I don't know if that's right.

Anyone have some firm numbers on this? I'd like to build it the right size to start, not retrofit later.

Erik

 

[tashasdaddy]

Re: Motor well size for dual 60s or 75s?

26 inch centers would only be 52 inches.

-----13-----I----------26-----------I-----13-----I
I_________26_________i__________26__________I
-----13----i-----13-----i-----13-----i------13-----

but it would be tight, to get 2 4cylinders in there. then you need room to work steering into it.

 

[erikgreen]

Re: Motor well size for dual 60s or 75s?

Yeah, that's why I figured 26 on each side too... if the motor can turn when it's 26 inches from another motor, it should be able to turn when it's that far from the hull, too.

But I'm just guessing.

Anyone out there actually have dual outboards on a boat? How far apart are your motors, and how much space is on either side?

Erik

 

[ondarvr]

Re: Motor well size for dual 60s or 75s?

I don't think there is any set measurement, it depends on the size and style of OB used and there is no clearance needed on the outside.

 

[erikgreen]

Re: Motor well size for dual 60s or 75s?

On the transom, you mean?

I'm guessing that clearance is needed ahead of the motor, in the well, for the powerhead to rotate toward the gunwale somewhat... in a full turn.

 

[jay_merrill]

Re: Motor well size for dual 60s or 75s?

I just measured my 1975 Thunderbird cathedral hull, which was originally designed for up to 200hp in a dual configuration. Given the motors available at the time, the transom would have had to be able to accomodate a pair of V4 engines.

The outside width of the boat at the top of the stern is 81 inches, and the width at the top of the transom (typical spalshwell arrangement) is 63 inches.

I believe that 26" on center is considered to be a standard measurement, but it seems to me that it is a bit tight for some installations. On my boat, it may need to be more like 29", because there is a lowered section in the center for a single engine installation. Since my bottom is a V design all the way to the transom, I would like to raise the mounting point for the twins a bit, and the lowered space would cause problems with that at 26".

 

[tashasdaddy]

Re: Motor well size for dual 60s or 75s?

eric i think most splash well are about 14-16 inches deep. basically the width of a 6 gallon fuel can. and you need about 5 inches of exposed transom on the inside to get the mounting bolts set.

 

[ondarvr]

Re: Motor well size for dual 60s or 75s?

On the transom, you mean?

I'm guessing that clearance is needed ahead of the motor, in the well, for the powerhead to rotate toward the gunwale somewhat... in a full turn.

Its the same, motors are different, what works for one won't work for another. Not all wells have enough clearance even for a single OB when that's what they were designed for, this is more an issue of the motor hitting when tilted up. Typically its one of those things where the person designing it says "I think we need about this much room".

 

[erikgreen]

Re: Motor well size for dual 60s or 75s?

Okay, thanks ondarvr.

Thanks for the measurements Jay.. I was going to try to go with a full width transom just to make sure I could use dual engines, but I think I can go with something like you have... about 63 inches of transom space, with some small boxes in the corners of the transom taking up the rest (I have a 96 inch stern width).

I think my motor well is sized ok, I can just widen it with the transom and be fine. It's sized for a single 70 right now.

I think whether this works or not is going to be more dependent on the motors I get than anything else.

I did figure out something on transom height... I'll be setting the height roughly equal to 20 inches from the bottom at the offsets where I expect to mount motors.. basically a single on this boat would be perfectly positioned, so I'll raise the transom height enough so twins at about 13 inches off center each will also be perfectly positioned.

Or should I space them farther out so the props both get still water?

Erik

 

[jay_merrill]

Re: Motor well size for dual 60s or 75s?

Just don't go too high on the mounting height. I wouldn't try to mount the motors so that the cav plate positions are even with the bottom at the raised portions of the "V," relative to the lateral positions. If you do, the outside motor may cavitate in turns. You also don't want your upper mounting bolts to go through the transom too close to the top of it, because that will transmit a twisting moment too far away from the splashwell or knee.

What motors are you thinking of using on the boat, and what is it rated for?

 

[Dhadley]

Re: Motor well size for dual 60s or 75s?

When they say 26" centers they mean from center to center of each outboard. That's about as close to each other and still have turning room. If the hull has lifting strakes you'll want to center the motors on them if possible.

 

[erikgreen]

Re: Motor well size for dual 60s or 75s?

No lifting strakes, the boat's rated for a max HP of 150.

So I should just stick with the 20 inch transom height you think? That'll put the props about 2-3 inches lower than than a single prop would be.

 

[jay_merrill]

Re: Motor well size for dual 60s or 75s?

Here's a pretty good discussion on the various factors involved in a twin engine installation - its from a Boston Whaler forum.

http://continuouswave.com/whaler/reference/twinEngineMounting.html

 

[erikgreen]

Re: Motor well size for dual 60s or 75s?

Good discussion Jay, thanks.

I did end up designing for 64 inches wide with height for a 20 inch shaft, and depending on what motors I end up with I may need to use mounting plates to adjust that.

Erik