I am preparing to repower my 1969 Alcan 370 with twin outboards. The choices are twin 115 hp, 135 hp or 150 hp. Does anyone know how much it will take to get my boat on plane?
Cheers,
Walt
How much power does it take to plane an Alcan 370?
Re: How much power does it take to plane an Alcan 370?
Or if someone can tell me at what speed their Alcan 370 gets on plane that would help.
Cheers,
Walt
Cheers,
Walt
Re: How much power does it take to plane an Alcan 370?
Hi Walt
You pose an excellent question, but one that is hard to answer. Even the simple question of what speed the boat starts planing is complicated. I will take a shot at answering the questions and hopefully others will chime in with their experiences.
How much power will it take to plane?
The original Alcan power was a pair of 170HP engines which the brochures indicated would propel the boat at 25MPH which is clearly planing. However, most boats with these original engines will not plane today. This is likely due to a combination of low output from the aging engines and all the additional weight from accessories and years of modifications. The outboards you suggest are comparatively low on HP. However the outboards have two advantages; 1, they will be very light weight which should help a lot; and 2, they will impose much lower drag compared with the typical stern drives.
At what speed does your boat plane?
If you look at the RPM-speed curve for Buster (http://www.alcanboats.com/newsite_wordp ... l_dynamics), you can see an inflection point at around 12MPH. This is well above the displacement speed for the hull, but it is hard to say the boat is on plane. At 12MPH, the boat is likely still in semi-displacement mode, where you can see the separation of water from the transom, but the hull is not yet fully supported by dynamic lift. I would estimate that the latter occurs at around 17 to 18MPH, but this is just a guess. The real lesson I take from that graph is that the least efficient speed to push the boat is 12MPH.
I would love to be able to tell you that one of the engine choices will give you the performance you want. The outboard has an advantageous configuration, but given that all of your choices are lower horsepower than the original engines it is hard to be confident. One last thing to keep in mind is that a significant factor that affects a boat's planing performance is the location of the boat's longitudinal center of gravity. Getting that right will make a huge difference.
I am hoping others can chime in with their re-power performance experiences. Let us know what you do.
Cheers
David
You pose an excellent question, but one that is hard to answer. Even the simple question of what speed the boat starts planing is complicated. I will take a shot at answering the questions and hopefully others will chime in with their experiences.
How much power will it take to plane?
The original Alcan power was a pair of 170HP engines which the brochures indicated would propel the boat at 25MPH which is clearly planing. However, most boats with these original engines will not plane today. This is likely due to a combination of low output from the aging engines and all the additional weight from accessories and years of modifications. The outboards you suggest are comparatively low on HP. However the outboards have two advantages; 1, they will be very light weight which should help a lot; and 2, they will impose much lower drag compared with the typical stern drives.
At what speed does your boat plane?
If you look at the RPM-speed curve for Buster (http://www.alcanboats.com/newsite_wordp ... l_dynamics), you can see an inflection point at around 12MPH. This is well above the displacement speed for the hull, but it is hard to say the boat is on plane. At 12MPH, the boat is likely still in semi-displacement mode, where you can see the separation of water from the transom, but the hull is not yet fully supported by dynamic lift. I would estimate that the latter occurs at around 17 to 18MPH, but this is just a guess. The real lesson I take from that graph is that the least efficient speed to push the boat is 12MPH.
I would love to be able to tell you that one of the engine choices will give you the performance you want. The outboard has an advantageous configuration, but given that all of your choices are lower horsepower than the original engines it is hard to be confident. One last thing to keep in mind is that a significant factor that affects a boat's planing performance is the location of the boat's longitudinal center of gravity. Getting that right will make a huge difference.
I am hoping others can chime in with their re-power performance experiences. Let us know what you do.
Cheers
David
Re: How much power does it take to plane an Alcan 370?
Hi Walt,
To add to David's info, Catherine Ann has the original 2 x 170 HP Volvo engines and does not reach planning speed. Although hull shape is identical to Buster (somewhat different than your Alcan 370) I estimate that my boat is 5000 lbs heavier than David's. Flybridge, teak decks, stainless and wood railings, residential fridge, generator, new cabin sheathing, etc . . . all add up. 17,000 lbs vs 12,000 lbs (or less). Side by side, my boat sits 2" deeper in the water than David's. The "floating cottage vs the speedster" !
All this to say that boat weight is as critical as hull shape to reaching planing speed. Your boat originally planed with about 340 HP. Before deciding on repower HP, it would be good to get an accurate weight of the boat and compare this to original specs.
Plotting Speed vs RPM for Catherine Ann vs Buster, looks like this:
To add to David's info, Catherine Ann has the original 2 x 170 HP Volvo engines and does not reach planning speed. Although hull shape is identical to Buster (somewhat different than your Alcan 370) I estimate that my boat is 5000 lbs heavier than David's. Flybridge, teak decks, stainless and wood railings, residential fridge, generator, new cabin sheathing, etc . . . all add up. 17,000 lbs vs 12,000 lbs (or less). Side by side, my boat sits 2" deeper in the water than David's. The "floating cottage vs the speedster" !
All this to say that boat weight is as critical as hull shape to reaching planing speed. Your boat originally planed with about 340 HP. Before deciding on repower HP, it would be good to get an accurate weight of the boat and compare this to original specs.
Plotting Speed vs RPM for Catherine Ann vs Buster, looks like this:
- Attachments
-
Re: How much power does it take to plane an Alcan 370?
Hi David and Rob,
Great information. Thanks for all the feedback. As far as the weight of the Skinny Dugan, I know that when I craned it from the transport truck onto my trailer it was 12, 400 lbs. Since then I sanded off about 100 lbs of paint
and removed the genset. the boat is probably now @12,000 lbs.
Here's a question: Outboard motor horsepower is rated at the output shaft I am told. What is the shaft output horsepower for your powerplants? The same as the motor spec or derated once it gets to the output shaft?
Cheers,
Walt
Great information. Thanks for all the feedback. As far as the weight of the Skinny Dugan, I know that when I craned it from the transport truck onto my trailer it was 12, 400 lbs. Since then I sanded off about 100 lbs of paint
and removed the genset. the boat is probably now @12,000 lbs.Here's a question: Outboard motor horsepower is rated at the output shaft I am told. What is the shaft output horsepower for your powerplants? The same as the motor spec or derated once it gets to the output shaft?
Cheers,
Walt
Re: How much power does it take to plane an Alcan 370?
Walt
Good point about rated horsepower. The stern drive engines are rated at the engine shaft and definitely lose something through the transmission. The manufacturers don't publish the transmission losses though, so I can only guess at somewhere around 5% or a little higher. The U-Joint operates at a fairly flat angle when running straight, so it probably doesn't absorb much power, but the stern drive takes 2 right-angle turns and uses helical-cut gears, so there is definitely some loss there. If the outboards are rated at the prop, then they have yet another advantage. In terms of real output, that would put the 150HP outboard within spitting distance of the 170HP original engines. The lighter weight and added hydrodynamic efficiency of the modern lower unit, may be more than enough to equalize the differences.
An interesting note about helical-cut gears. Many years ago I built a high-performance Volkswagen engine for a Dune Buggy. Replacing the helical-cut cam drive gear with a straight-cut gear was estimated to add 2HP to the engine output. I didn't have before and after measurements, so I can't attest to it working, but I can sure tell you it was noisier.
If it weren't so expensive to conduct experiments I might be tempted to try it out.
David
Good point about rated horsepower. The stern drive engines are rated at the engine shaft and definitely lose something through the transmission. The manufacturers don't publish the transmission losses though, so I can only guess at somewhere around 5% or a little higher. The U-Joint operates at a fairly flat angle when running straight, so it probably doesn't absorb much power, but the stern drive takes 2 right-angle turns and uses helical-cut gears, so there is definitely some loss there. If the outboards are rated at the prop, then they have yet another advantage. In terms of real output, that would put the 150HP outboard within spitting distance of the 170HP original engines. The lighter weight and added hydrodynamic efficiency of the modern lower unit, may be more than enough to equalize the differences.
An interesting note about helical-cut gears. Many years ago I built a high-performance Volkswagen engine for a Dune Buggy. Replacing the helical-cut cam drive gear with a straight-cut gear was estimated to add 2HP to the engine output. I didn't have before and after measurements, so I can't attest to it working, but I can sure tell you it was noisier.
If it weren't so expensive to conduct experiments I might be tempted to try it out.
David
Re: How much power does it take to plane an Alcan 370?
Hi Walt,
There are so many variables in this equation . . . it is impossible to solve from the sum of the bits and pieces of information. There have been so many Alcans repowered (although none that I know of with outboards) it might make sense to look at what others have done and what worked or did not work for these other boats. Your plan does have significant differences with a major reduction in underwater drag between the outboards and the original outdrives and a major shift in longitudinal centre of gravity towards the bow with removal of inboard engines and generator. You are going to have a major "cargo hold" back there, to fill with something . . . maybe a huge bank of batteries ? If you look at pictures of Buster, or any other Alcan/Alwest boats on plane, the boats always maintains a bow up / stern down attack angle. I don't know what happens if you lighten the stern enough to approach a level plane like a more modern hull.
Rob
There are so many variables in this equation . . . it is impossible to solve from the sum of the bits and pieces of information. There have been so many Alcans repowered (although none that I know of with outboards) it might make sense to look at what others have done and what worked or did not work for these other boats. Your plan does have significant differences with a major reduction in underwater drag between the outboards and the original outdrives and a major shift in longitudinal centre of gravity towards the bow with removal of inboard engines and generator. You are going to have a major "cargo hold" back there, to fill with something
. . . maybe a huge bank of batteries ? If you look at pictures of Buster, or any other Alcan/Alwest boats on plane, the boats always maintains a bow up / stern down attack angle. I don't know what happens if you lighten the stern enough to approach a level plane like a more modern hull.Rob
Re: How much power does it take to plane an Alcan 370?
Well Gentlemen,
It time that I open another string on the tech tab called "repowering an Alcan 370 with Twin 150 Yamaha Outboards" - I bought the motors. They're old and used, but pretty. Stay tuned for pictures on a new topic!
Thanks for all your feedback. The truth is I am not in a hurry, so plane or not, we'll enjoy the journey.
Cheers,
Walt
It time that I open another string on the tech tab called "repowering an Alcan 370 with Twin 150 Yamaha Outboards" - I bought the motors. They're old and used, but pretty. Stay tuned for pictures on a new topic!
Thanks for all your feedback. The truth is I am not in a hurry, so plane or not, we'll enjoy the journey.
Cheers,
Walt
Re: How much power does it take to plane an Alcan 370?
Hi Walt,
Sounds like a great project . . . that many have talked about but no one (that I know of) has embarked on.
Here is a place to start on motor mounting bracket ideas http://www.youtube.com/watch?v=2GORpt8Cybw.
Strengthening the transom to take this different load will be interesting.
The Alcan 370 blueprints (on the website) should be a good place to start.
Rob
Sounds like a great project . . . that many have talked about but no one (that I know of) has embarked on.
Here is a place to start on motor mounting bracket ideas http://www.youtube.com/watch?v=2GORpt8Cybw.
Strengthening the transom to take this different load will be interesting.
The Alcan 370 blueprints (on the website) should be a good place to start.
Rob