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Old 04-15-2015, 09:52 PM   #1
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Converting large gas to small diesel

Greetings- I know there is a thread on this subject earlier on and just not able to locate it. I apology.

Speaking to conversions. In a earlier thread the subject of converting a larger yacht reflecting a very favorable low price as demonstrated in this ad with gas engines.
Were I a few years younger and had the opportunity as I do now to have available two 58 hp diesel engines. One is the current Perkins 4-154 with a 3;1 gear coming out of our boat and a purchased backup to this engine in inventory, a Westinbeke W-58 which is the same hp using the same basic 4-154 Perkins modified by the Japanese company (Not Mazda but they used the modified engine in their trucks during the 70s. It too has a 3;1 gear. While the Perkins has a velvet drive, the Westinbeke has a different gear manufacture.

As the Velvet drive can be direction reversed with a simple flip it would be simple to have counter rotating gears. 100 hp through these 3;1 gears would allow throwing 21X16 wheels and would be on the surface sufficient to move the hull at hull speed or 6.5/7 knots. (Not scientific, just using the current ratio and wheel on a 27 foot single screw boat of SD construction.)

So, you would have all the accommodations, in a fiberglass hull of known manufacture being driven by economical power at a purchase price of a new 16 foot skiff and outboard of 70 hp. (Assuming a ton of individual owner involvement with the conversion)

It is not the intent to dredge up old debate on various counter thoughts, just wanted to present an example of a backyard project to accomplish a level of boating not readily available to a like thinking DIY type of boater.

1973 41 Fiberglass Chris Craft.

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Old 04-15-2015, 11:19 PM   #2
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That would be great Al if you'd turn up the stern like that of a flat bottomed wood rowboat. Would need to basically turn it into a full displacement hull to have any real economy. Even a barge has that advantage. Otherwise it's a great idea.
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Old 04-16-2015, 12:18 AM   #3
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Al I do not disagree with the idea and have considered a similar project but as a purely practical matter the ingredient most often overlooked is a location to do the work. If a person has the land and ability to transport it for reasonable cost it is viable. Working in a DIY boat yard many miles from home brings about its own set of challenges.
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Old 04-16-2015, 07:06 AM   #4
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IF you can weight the boat at a yard , usually 3HP per ton (2240lbs a ton) will give good displacement speeds , with some weather /wind wave reserve.

If your Perkins has the ability to create that HP , on a long term basis , you are ready to go.

Old boats tend to gain lots of weight over time , so an actual weight is worth the cost and effort.
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Old 04-16-2015, 07:23 AM   #5
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Quote:
Originally Posted by Al View Post
So, you would have all the accommodations, in a fiberglass hull of known manufacture being driven by economical power at a purchase price of a new 16 foot skiff and outboard of 70 hp. (Assuming a ton of individual owner involvement with the conversion)

Sweat equity can be a good thing... if it doesn't hurt, too much afterwards. Physically, I mean.

Looks like a nice boat. I'd first compare the cost of your diesel conversion to the cost of doing nothing, i.e., using the existing Ford gassers as they stand. For short-distance and slow-speed cruising (if you do that), the labor for a diesel conversion might not gain much, if anything.

Then I'd also compare the cost of dropping in a couple new Crusader 6.0L gassers if the Fords crap out.

Either gas option probably means no need to replace a gas genset...

All the otherwise useful advantages of diesel notwithstanding...


A previous boat neighbor converted an '80s 35' Bertram from gas to diesel a few years ago, with a pair of new 5.7 Cummins Bs. Really liked it, but it cost about $100K for the conversion job, and the boat is now for sale, not selling for even the amount he put in it. That's a different situation from what you describe, of course... but the result was that he started with a boat of $X value, spent a boatload, and ended up with a boat of $X value.


Maybe it'd all depend on how much fun you'd be having investing your sweat (knowledge, skill, ingenuity, physical effort, etc.).



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Would need to basically turn it into a full displacement hull to have any real economy.
How much improvement could there be? A half-gallon/per hour/per engine? One? Or...?

-Chris
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Old 04-16-2015, 07:54 AM   #6
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Often I look at the old style motoryachts with a gleam in the eye and remembrance of the finer days of yachting.

I could very easily see a full restoration to be eye candy yet repower with some small diesels for longer distance cruising.

Like many of the semi displacement vessels others here own, running her with small efficient diesels would be satisfactory in my mind. The effort to change hull shape wouldn't be warranted. If I wanted ultimate fuel economy, I would go back to sail.

For most of us, life is practical, not an exercise in fiddling with stuff till it meets some perception of perfection.
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Old 04-16-2015, 10:42 AM   #7
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Ranger wrote;
"How much improvement could there be? A half-gallon/per hour/per engine? One? Or...?

Opinions vary on that but roughly I'd say 1.5 on a SD hull typical of psneeld's Albin or an IG and 2 times as much as a planing hull ... like the CC in this post. A narrow boat would be better than a wide one. With the CC one would half the fuel by turning up the stern. That has nothing to do w the efficiency of power .. just the drag of the hull. FF mentioned 3hp per ton for FD .. that's for a really slick FD hull or just slower speed. My FD hull is at 5hp per ton and won't make hull speed. At least I don't think. The lowest hp per ton I've seen for a mono hull is 2hp per ton. Even after a converted stern to FD the CC's hull won't be as efficient as a typical FD. And if the turned up stern turns up abruptly there will be less gain at 1 knot below hull speed as it would have if the stern curved up gradually over a longer section of the hull. Barges (the typical double ended variety) have a very abruptly turned up stern but the advantages in lower drag must be very substantial indeed as I've never seen one w a stern typical of a basically right angled bottom to transom angle.

But it would be a lot of work justifiable only if most all of the work could be done by the owner. I've thought through the years that it would be easier to convert a wood boat than a FG but the shape of the hull may more of a deciding factor.

Opinions vary probably more than reality on this. TAD (our resident NA) I'm quite sure thinks the drag of a SD hull is much closer to a FD at cruising speed (1 knot below hull speed) than I do and a conversation about this on boatdesign.net had a lot of varying opinions. But if one were to row a flat sterned skiff and then a propper row boat (like what is called a "pulling boat" your mussels in your body and the very sluggish speed (or lack thereof) would be very convincing. I've seen people ballast the bow of a skiff so the stern is out of the water and row it backwards to basically turn it into a FD hull. Must be a bugger to keep going straight.

But the conversion of the CC in question may make it steer better especially in following seas and there would be other advantages and disadvantages. But because it would be a big job I may never see one done. And the end result would never be as good as a proper FD hull. Just like converting a sailboat to a powerboat wouldn't be as good as a powerboat ... ha ha but would probably be more efficient one knot below hull speed.

But if one had the skills a conversion could be an economical way to obtain a large trawler-like boat for very little money w almost the same fuel efficiency of a FD hull. For understandable reasons big gas powered cruisers are hardly worth anything now and w the right surrounding circumstances one could buy a nice boat for really cheap. But leaving the square sterned CC above as is would make the conversion probably not worth doing. But w a really light boat like one of plywood the conversion would become much more beneficial w the hull left as is. HP per ton - half the weight - half the power required.
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Old 04-16-2015, 11:05 AM   #8
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Quote:
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Opinions vary on that but roughly I'd say 1.5 on a SD hull typical of psneeld's Albin or an IG and 2 times as much as a planing hull ... like the CC in this post.

But it would be a lot of work justifiable only if most all of the work could be done by the owner.

Hmmm....


Even for a 1.5 GPH improvement... that'd sure be lots o' work, no matter who does it. Lot o' not-using-the-boat time, for what I'd consider an inconsequential gain. (Being more into boating than I am into boat building.)

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Old 04-17-2015, 01:21 AM   #9
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Thanks Guys, good chatter. Now to sell those two Perkins/Westenbeke diesels and continue on with the 4-346 installation. (Starts tomorrow with boat pulling)
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Old 04-17-2015, 07:12 AM   #10
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For some folks remember time marches on in the engineering world.

Gas engines used to run 10HP/ GPH but the desperation to meet CAFE requirements has improved things.

Some gassers can produce 14HP/GPH about where older or underloaded diesels operate.

Gas is cheaper , and spark plugs are $4.00 each not $1000 for modern injectors.

The new gassers are lighter than the old units , another help for an older boat.

Sure the interior volume of an old plaining boat will be near a Full Disp boat ,

but the ride, seaworthy, sea kindly and low speed steerring may be GONE

Best to operate the boat on a cruise for a while traveling at the Sq Rt of the LWL , to see how the vessel operates, as a displacement vessel...

Props ,

a rough rule of thumb is an inch of pitch at 1000RPM gives one Knot of speed ,,,,,IF the prop had no slip.

So the proposed 16 inch wheels would be near 8K with the usual 50% slip.

Not a bad place to start , but remember you would need to cruise at 3000RPM to get 1000RPM with a 3-1 gear box.

Probably will be lots of noise and vibration, and a good sized fuel burn.
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Old 04-17-2015, 01:44 PM   #11
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Quote:
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Opinions vary probably more than reality on this. TAD (our resident NA) I'm quite sure thinks the drag of a SD hull is much closer to a FD at cruising speed (1 knot below hull speed) than I do and a conversation about this on boatdesign.net had a lot of varying opinions.
Below is a comparison of three resistance curves, a full displacement, semi-displacement, and planing hull. All of them are 28' overall, but otherwise completely different. Waterline length, beam, displacement, entrance angles, and transom immersion are different.

Note that the full displacement (Ironbark) and the planing hull (Red Cedar) have the same resistance at around 7 knots. The semi-displacement hull (Douglas Fir) and the planing hull have the same resistance at 4 and 15 knots. The displacement and semi-displacement boat have the same resistance at 6 knots.

This data is derived from tank testing families of similar hull forms, altering just one dimension to create each new family member.

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Old 04-17-2015, 03:07 PM   #12
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Thank you for the data points Tad.
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Old 04-17-2015, 10:33 PM   #13
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Thanks Tad, that was one of my favorite Woodenboat articles on the subject. Can't believe it was 18 years ago.

Seriously, that article made me consider Westlawn and I made a visit to the Landing School as well.
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Old 04-18-2015, 02:49 PM   #14
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Can't believe it was 18 years ago.
You and me both.....

Quote:
Seriously, that article made me consider Westlawn and I made a visit to the Landing School as well.
I know it's a minefield to start advising people, but if anyone is interested in designing boats professionally I would say go to university and get a degree in Naval Architecture. There are lots of lean times for yacht designers and a NA degree will keep the wolf from the door. Besides yacht design has become far more technical and specialized than either Westlawn or the Landing school can cover.
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