Bolger Diablo new build

[TimmS]

Hi All!

I’m building a Bolger Diablo for my own (plus wife, daughter, and dog!) use after taking some years off from building boats. If any of you remember me, hello again.

A friend built one years ago, so I have experience with the design and its performance. I’ve chosen to switch up the building process by building her right side up, sort of Japanese style. I wanted to have the bottom attached to a solid base to keep it perfectly flat and it also allowed to to keep the bulkheads loose so that I could take them out to tape the hill joints with glass and I don’t have to commit to installing them until I work out the seating and possible small bow deck. Odd, I know, but I’m glad I did.

My question for anyone who has experience with this design or outboard powered boats in general is regarding my motor. It is a 15hp Johnson 15” shaft. It’s over 20 years old, but basically new. The Diablo as designed has about a 18.5” transom. I’m guessing this was intended for a 20” motor? I would like to make this motor work and also build a boat that will accommodate another common motor in the future, possibly electric. Below are the options I’ve thought of. Feel free to comment on any of them or add your own idea. Thanks in advance.

1) cut down transom. A little less than 2” would get the anti-ventilation plate in line with the flat bottom. I’m not crazy about losing freeboard. Could be added back when/if I change motors.

2) add an extension to the motor. They can be had used for reasonable money but in rough shape and wrong color. Adding 5” seems like a lot.

3) proceed as is and see how it works.

4 start out with a different motor. I’d like to avoid this one.

[Dave Hadfield]

If you’re going to lower the transom then it might be a good idea to install a full-height bulkhead a foot forward.

When you yank the power off, and generate an overtaking wave, you don’t want it washing into the boat.

I don’t know if that’s already in the plans.

[Boatsbgood]

I dont know the boat, but with 3 and a dog is 15hp enough? If youre confident and staying with that motor I've an extension housing you can have for shipping (USPS flat rate box). You then need a drive shaft (available on ebay, cheaper than short ones) a water tube or short extension, (also ebay) and a long shift actuator, (guess where?). Dont have to pull powerhead if you use an extension water tube, and you'll want to start with a fresh pump impeller anyway so not really much work. I'd not shorten the transom, but lengthen the shaft even if you have to build up the transom for optimal prop/plate height.
Brian
ETA the extension housing as a Johnson, white, tidy, I think very little used paint intact.
Edit to add one more: That shop!!!!!

[TimmS]

If you’re going to lower the transom then it might be a good idea to install a full-height bulkhead a foot forward.

When you yank the power off, and generate an overtaking wave, you don’t want it washing into the boat.

I don’t know if that’s already in the plans.

Great point, but with tiller steering that bulkhead couldn’t really be any higher

[TimmS]

I dont know the boat, but with 3 and a dog is 15hp enough? If youre confident and staying with that motor I've an extension housing you can have for shipping (USPS flat rate box). You then need a drive shaft (available on ebay, cheaper than short ones) a water tube or short extension, (also ebay) and a long shift actuator, (guess where?). Dont have to pull powerhead if you use an extension water tube, and you'll want to start with a fresh pump impeller anyway so not really much work. I'd not shorten the transom, but lengthen the shaft even if you have to build up the transom for optimal prop/plate height.
Brian
ETA the extension housing as a Johnson, white, tidy, I think very little used paint intact.
Edit to add one more: That shop!!!!!

Thank you! The boat my friend built had a 9.9 on it and was quite fast. We’re not trying to break any records and our lakes aren’t very big so 15 should serve us well. If I ever bought a new motor I would probably go 20 or 25. Your offer is very kind, I’ll be in touch if I decide to go that route.

[MushCreek]

18-1/2" is kind of an oddball height. I would say it's definitely too high for a short shaft motor. I always used 20" fr a long shaft; 15" for a short shaft, except in high performance boats, where you go as high as you can get away with. I guess there would be no harm in trying it as-is. You'll know pretty quickly if it's too tall for the short shaft motor. If it were me, I'd go to a long shaft rather than cut the transom down.

I've considered building a Diablo if I ever decide to have another power boat. How stable is it sitting still? Comfortable enough to stand up while casting?

[Pateplumaboat]

If you’re going to lower the transom then it might be a good idea to install a full-height bulkhead a foot forward.

When you yank the power off, and generate an overtaking wave, you don’t want it washing into the boat.

I don’t know if that’s already in the plans.

Pyson writes on page 74 in his book ISBN 0-07-155966-3 "Build the new Instant Boats" mentioning Phil Bolger:

"On the Diablo plan printed last summer, the motor drawn is a short-shaft type. This was cerelessness on my part, in tracing from the wrong template. Diablo is designed for long-shaft motors. She has a 19 1/2-inch transom, that ought not to be cut lower as there is no slop well".

My interpretation: If one would use a short-shaft outboard one has to build a slop well for Diablo, which I think would be relatively easy to make.

[hawkeye54]

No 'contribution' to the boat build here - just saying I , too ,love that shop and the arches over the windows on the right of the photo ! !




Rick

[Breakaway]

I would get the spec for the exact model and year engine you have.

The shaft lengths of 15-, 20-,25- and 30-inches are nominal measurements. Engines, except for tiny ones, have mounting brackets with several mounting holes so you can adjust (or select an exact) the mounting height. Using the highest mounting hole, your 15-inch engine might just be OK on an 18.5-inch transom.

Kevin

[Boatsbgood]

This motor is an OMC 9.9/15, 2stroke, maybe 80 lbs? Screw-clamp mounts. One hole for bolts at bottom of bracket.

[TimmS]

Thanks for all of the great input. I’ve attached another photo that shows the actual motor on the transom as designed. I don’t think it would work as is and I'm considering adding a splash well. I’m exploring the well proportions with hot glue and cardboard. I like how pristine and compact this motor is without adding a shaft extension, and keeping the weight low is always good.

[SalsDad]

We’re not trying to break any records and our lakes aren’t very big so 15 should serve us well. If I ever bought a new motor I would probably go 20 or 25.

I had a Diablo for a number of years, and did some experimentation with transom height. She was used year-round for commuting, cargo, and joyriding in the lower Kennebec River: miles pounding into short steep water, sometimes racing down near-breaking ocean swell, and also frequently skimming over skinny water, above mudflats, usually on an outgoing tide.

I bought the Diablo used, heavy, and badly built (when it was time for repairs, I discovered a layer of uncured goop between ply and glass with an old 15HP long-shaft. The 15 proved nearly unable to get onto a plane with more than 5-600 lbs passengers and cargo, and the long shaft required far too much depth.

When it was time for new power and a rebuild of the hull and interior, we chose a 25HP short shaft 4-stroke. I left the transom top where it was, and cut a wedge-shaped "tunnel" out of the bottom, about 6" wide, 12" long, and tapered from 0 to 2.5" at the transom, so the short-shaft motor was permanently "jacked". Performance was beautiful, planing easily with anything but a very heavy cargo. In a pounding chop, the boat can take a lot more than you can.

Other modifications at that time: Interior changed to a pair of longitudinal seats/float chambers with a deep well down the center, and a raised sole forward. This made her 80pct self-bailing, with rainwater collecting only along the centerline. Lower stem and the first 18" of the bottom were armored with aluminum plate, to facilitate landing on ledges. (nose up til you bump a ledge near the water line, then use the engine to push up onto the rock for a secure landing)

As to the transom height/slop well question, even cut down to 16", I wouldn't worry too much about water over the stern on your not-so-big-lakes. You can always add a little slop well later.

Diablo is a really great boat - have fun!

[Breakaway]

Thanks for all of the great input. I’ve attached another photo that shows the actual motor on the transom as designed. I don’t think it would work as is and I'm considering adding a splash well. I’m exploring the well proportions with hot glue and cardboard. I like how pristine and compact this motor is without adding a shaft extension, and keeping the weight low is always good.

That's too high.

Kevin

[WX]

I built one back in 92, we put a 25hp outboard on it and it flew along.

[TimmS]

I had a Diablo for a number of years, and did some experimentation with transom height. She was used year-round for commuting, cargo, and joyriding in the lower Kennebec River: miles pounding into short steep water, sometimes racing down near-breaking ocean swell, and also frequently skimming over skinny water, above mudflats, usually on an outgoing tide.

I bought the Diablo used, heavy, and badly built (when it was time for repairs, I discovered a layer of uncured goop between ply and glass with an old 15HP long-shaft. The 15 proved nearly unable to get onto a plane with more than 5-600 lbs passengers and cargo, and the long shaft required far too much depth.

When it was time for new power and a rebuild of the hull and interior, we chose a 25HP short shaft 4-stroke. I left the transom top where it was, and cut a wedge-shaped "tunnel" out of the bottom, about 6" wide, 12" long, and tapered from 0 to 2.5" at the transom, so the short-shaft motor was permanently "jacked". Performance was beautiful, planing easily with anything but a very heavy cargo. In a pounding chop, the boat can take a lot more than you can.

Other modifications at that time: Interior changed to a pair of longitudinal seats/float chambers with a deep well down the center, and a raised sole forward. This made her 80pct self-bailing, with rainwater collecting only along the centerline. Lower stem and the first 18" of the bottom were armored with aluminum plate, to facilitate landing on ledges. (nose up til you bump a ledge near the water line, then use the engine to push up onto the rock for a secure landing)

As to the transom height/slop well question, even cut down to 16", I wouldn't worry too much about water over the stern on your not-so-big-lakes. You can always add a little slop well later.

Diablo is a really great boat - have fun!

Id love to see photos of those details if you have any! So far all of my interior is loose and I have been pondering seating options. Two long benches came to mind...they would be especially nice it they were wide enough to lie down in the sun on! I am leaning toward lowering the transom without building a well, at least at first. The attached photo shows how little effective transom would be lost since the motor blocks a lot of it. Its hard to imagine a situation there this small area made a big difference in safety.9A189AB4-8387-4772-8FD7-BC93A7D49BE1.jpg

[Wayne Poulsen]

Hi
I had a diablo and was just yesterday thinking about building another.
A lot depends on how you intend to use her.
I used long shaft motors , 5 and 20.
If you are in flat water thats one thing, but pulling lobster pots over here in a chop I would not build without an effective slop well.
That extra little bit of area when you cut down the transom does matter...it reduces the freeboard of the WHOLE boat by that amount!
In repeated waves the dynamic is this after that first slop sinks the boat a little every successive one gets bigger !
I would also make sure to have plenty of bouyancy near the transom , Bolger recommends 4 cuft.
Wayne
Fremantle
PS outside the scope of this discussion but should I build, I would add 3-4 foot out the back, either as full hull extension or a "full width pod/swim platform".
I also would consider Diablo Grande.

[SalsDad]

ChatGPT's sonnet to Diablo:

Phil Bolger's Diablo, sleek and daring design,
A boat that cuts through water like a knife,
With lines that are both elegant and fine,
A joy to sail, a beauty in our life.

Designed to be both fast and versatile,
This boat can weather any kind of weather,
With its sharp bow and deep V-hull,
It glides through waves like a bird of feather.

The brainchild of the master of the craft,
Phil Bolger, who understood the sea,
He built a boat that would outlast,
And be the envy of all to see.

So here's to you, Diablo, boat of fame,
A vessel built to sail in any game.

[WX]

This is the one I built.
20230130_143329.jpg

20230130_143306.jpg

[spirit]

In your photo in #11 it looks like the transom is only 0.5 inches thick. Is that correct?
Phil Bolger often specified a thickness of 2 inches for transoms...

[TimmS]

In your photo in #11 it looks like the transom is only 0.5 inches thick. Is that correct?
Phil Bolger often specified a thickness of 2 inches for transoms...

You are correct, it’s currently only 1/2”. It’s just the outer shell so far. I’m trying to finalize transom height, well (or not), seats, etc before adding structure to the inside of the transom. At this point it’s still possible for lots of things to change.

[Scrib]

Your outboard as pictured looks to be about 2" too high. As mounted you'll get turbulence around the cooling water intake screen which will lead to overheating issues. The cavitation plate needs to be down in line with the bottom of the boat. It could be a little bit higher than the bottom or a little bit lower but it really should not be as you are showing it. Lowering the engine will most likely cause a need for a splash well. Might as well build it in now while it's easy and I'd guess you could build it with scrap pieces so the cost should be minimal. You're doing good work, keep it up!

[Oldad]

Your outboard as pictured looks to be about 2" too high. As mounted you'll get turbulence around the cooling water intake screen which will lead to overheating issues. The cavitation plate needs to be down in line with the bottom of the boat. It could be a little bit higher than the bottom or a little bit lower but it really should not be as you are showing it. Lowering the engine will most likely cause a need for a splash well. Might as well build it in now while it's easy and I'd guess you could build it with scrap pieces so the cost should be minimal. You're doing good work, keep it up!

as Scrib and Breakaway have pointed out, you have the motor mounted too high. I think, ideally the cav plate should be 1/2 inch below the bottom of the transom.

[SaltyD from BC]

I had a Diablo with a 25 hp 2 stroke Merc it flew it was pretty sporty borderline sketchy with just me. I ended up getting a 15 hp 4 stroke Suzuki and I figured that was about perfect. These hulls are very easily driven.

So, with the Suzuki it was a short shaft so I was in the same predicament as you. I ended up just cutting a notch barely wide enough to mount the short shaft motor lower. (the Merc was a long shaft, 20" it worked perfectly as designed). I got by with the notched transom but was reminded daily that I really should have found a long shaft. It shipped a bit of water ever time you went from a plane to stop when the wake caught up to you. And it was always a bit of a worry. So, it will work but not the best option.

[TimmS]

Your outboard as pictured looks to be about 2" too high.

Thanks, that's the exact reason I started this thread. If you read through the comments there has been great discussion about what to do about it.

[TimmS]

I had a Diablo with a 25 hp 2 stroke Merc it flew it was pretty sporty borderline sketchy with just me. I ended up getting a 15 hp 4 stroke Suzuki and I figured that was about perfect. These hulls are very easily driven.

So, with the Suzuki it was a short shaft so I was in the same predicament as you. I ended up just cutting a notch barely wide enough to mount the short shaft motor lower. (the Merc was a long shaft, 20" it worked perfectly as designed). I got by with the notched transom but was reminded daily that I really should have found a long shaft. It shipped a bit of water ever time you went from a plane to stop when the wake caught up to you. And it was always a bit of a worry. So, it will work but not the best option.

Thanks, great real world experience!

[WX]

Don't forget the spray rails.

[Wayne Poulsen]

hows it going TimmS?