I have been buying plans from Glen-l for over 32 years. starting with my first hydroplane at 14. A glen-L super titan.
I've owned a set of union jack plans, they got ruined on my houseboat from water. I also owned two sets of the boat I am building now...on this thread. I bought a copy of the coastal cruiser. And various others, such as the sweet 16. never had any problems till now.
I have an issue with the present plans. The problem is that the engine sizing is absolutely wrong for the shaft angle and prop sizes listed. the issue lies with the fact that there is no engine(unless you are from the 1930-40's) in the specified size range that can turn a 24 inch diameter prop. That is, it cannot be done with the called for engine of between 20-50 hp.
there are two mistakes made on the plans from what I can tell...and they are costly ones for me.
1. the "motor" shown in the plans set is under 24 inches high scaling directly form the plans at 1:1 as stated. showing lots of room above the engine in the drawings.
Unfortunately this is absolutely impossible. the smallest diesel out there in the specified hp at present and past has a height of about 27 inches from pan to top. most being around 29-30"
engines in the 50 -65 hp range are at least 29-35 inches high.
case in point. I am looking at a Volvo md 30, 65 hp and a bukh DV 20 , 20 hp, both are 29 inches high.
normally this isn't an issue since the FM tug has about 32 inches of height in the engine rm IF you don't count the cabin sole framing.
2. the second problem will tie into the first but bear with me...
If you want to turn that large a fan(24 inch diameter!) which I do, or at least say 20 inches. there is no solution with the gearing available for a standard gearbox running a diesel of 50 hp, to turn that big of a prop. The RPMS are just too high on modern diesels.
so the issue is that in order to turn this 20-24 inch dia prop specified, which would result in the most efficient use of the vessel and do what it is intended and designed for, you would need a gear reduction in the area of about 4:1. But, there just isn't any such animal. the best I could find was 3:1.
so doing the math using the BMC 1.5 litre I am looking at, along with two others in similar rpms ranges:
3500 rpms/3 = 1167 rpms.
This does not develop the torque required to turn a 20-22" prop. a true torque curve would indeed be in the 750-850 rpm range. that would require 3500rpms/4 = 875 rpms! not bad. bordering on the right shaft speed to turn a large prop.
ok, so lets connect the dots.
There are only two ways I know of to achieve this required torque and rpms.
A. to use a customized reduction gear. These price out at more than the costs of the finished hull. and by far more than the $1500.00 I am spending on my used diesel, which comes with a 2:1, a 2.5:1 or 2.4:1 reverse gear. (there are not any 3:1 gears on any engine I have seen as of yet in the Hp range needed, although I know "slug" off this site used a 3:1 on his Kubota, I dare not ask what he paid.)
todays diesels are high speed low torque for small props which fit into sailboats, sadly 50 hp gearboxes are not made in 4:1 ratio's.
or
B. find an alternative way to gear the prop shaft down.
one way to solve this is using hydraulics, but it is costly, since you need to go to heavy hydraulics and pumps, just to get 33.9 hp from a standard charr-lynn motor. again this is not financially viable. and makes no sense mechanically since you must use a 40 hp and up diesel to make it work without losses.
the other way is mechanically gear it to use a type of speed reducer or belt or polychain system. I have looked into all three of these.
now...In order for these to be used, you have to match the shaft angle. and have a jack shaft which runs the prop.
What handles the thrust of the prop? You need to jury rig some form of bearing system which can handle the incredible torque created, otherwise the shaft would screw itself right through the shaft log. and into the hull...ok its a bit overstated but you get the point.
herein lies the BIG problem.
In order to get a viable system lets say for instance using a polychain drive, you need to raise the engine up off its designed engine beds, in order to create the proper shaft center distance. That is, reasonable if you use a 1.5 inch shaft , and you want to gear that to say 2:1(taking into account there is already a 2:1 gearbox/reduction on my present diesel) you need to make those centers as far apart as possible to put the largest feasible pulleys on the system. on top of that you must take into account the force of the "pull" on the drive shaft from the gearbox. usually this can be handled by roller bearings, which means replacing the standard ball bearings or if possible using a pillow block. The centers and pulley diameters are determined by computer. The smallest input diameter for the shaft @ 2:1 reduction is about 4.75" and about 9"+ for the drive pulley, the lowest center feasible for my application comes in at around 10-14" of center add the lower half of the drive pulley to the center distance to determine the height to raise the engine, and you will find some BIG issues!
ok...bottom line; this means that the engine must raise at least that amount from its present position in the plans upwards 10 14" to allow for the center distance.
HERE is where it gets crazy!
you cant raise it. not without going through the accommodations floor and increasing the shaft angle. too far. which is butchering the design as far as I am concerned.
The plans show the height of the engine to be less than 24 inches in the plans. that means someone did not do their homework in the plans design stage. The prop is actually larger than the engine on the plans.(???) To raise the engine does not seem an issue if you are ok with going through the floor and perhaps adding ballast to accommodate the weight change, above the LCB.
So, that's it. I wrote to Glen-l regarding this matter yesterday. I am asking to have a refund for the plans since I cannot practically make this happen without spending more than should be spent to get the gear reduction necessary to tow my accommodation's barge through nasty weather if necessary. It is the Great lakes after all.
However- I am going to look at one other option. a gear reducer.
This is a cast iron reduction box, that has very close "helical" centers and might allow my engine to be raised only 5+/-" thus not creating an engine protruding through into the accommodations space above. the problem again is costs. they are not cheap at around 1400.00 U.S. plus 550.00 for the coupler. ok, its more than my engine. which kind of irks me somewhat.
This adds enough extra "save" time to put my project back a full year since I would need to fork that out in the spring.
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so there you have it. a small mistake in the plans which turns out to be a big one for me, and had I have seen that error sooner\(not sure how I could have?) it might possibly not cost me big bucks.
I would probably prefer to restart a different steel boat -one that does not need a large reduction. I.e. a Union Jack perhaps.
But I will await to hear back from Gayle- I'll keep you posted...
cheers
Doug
is there some intended use for them or are they just to see if the frames are aligned on the strongback?
(You know, a flux capacitor and a Mr. Fusion mini reactor doesn't take up as much room... 