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Old 24-03-2008, 09:44   #31
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I am with you Doug If I wanted to spare money and wanted the maximum speed ( is minimum weight )I would go that route it is easy enough to at a later date install retractable motors .

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Old 24-03-2008, 18:05   #32
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Originally Posted by schoonerdog View Post
I'm going to start a big disagreement here, but were I to have the option I would think about retractable yamaha 9.9 outboards off of inner cockpit and save myself 1000 lbs of weight. It would be a similar setup to the PDQ 36, isolated engines, isolated fuel tanks, much lighter weight, can hoist and replace on any island with mail service. Actually with the PDQ, you simply use your mainsheet to hoist up the engine straight up, my wife can do it. Any maintanence you need to do is on an engine that's hanging in mid air, couldn't be easier. The engine is easily retractible and you have no metal in the water when fully up. No prop fouling, easily to repair broken prop, etc. A good yamaha enduro two stroke is almost as reliable as a diesel, very fuel efficient and is about $2700 to replace. I'd do that as a short term solution and in five years I'd use the money I saved from the diesels to make an easy upgrade to a retractible electrical engine ala african cats. The outboard engine wells would serve for easy conversion to retractible electric drives. Look at pictures of a PDQ 36 and see their setup and it will be clear what to do. You can see the outside of the engine well with this picture http://www.2hulls.com/usedcatamaran-...36_Cockpit.jpg. It makes economic sense as well, in 5 years the price for the low weight batteries will fall by half, the price for solar panels will fall 4 fold, and the price of gas and diesel will double.
I'm pretty sure outboards in wells are an option on the Wilderness 1230.
I agree, except I would go with 20hp Honda's.
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Old 24-03-2008, 18:31   #33
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I agree that outboards have certain advantages but I prefer the diesel route.

One thing that I have not yet come to terms with is that many designs that offer the option of outboards or diesel usually suggest the 9.9 ob's or 30 horse diesels. Are the diesels way overpowered or the ob's woefully inadequate in all but very benign conditions.
Even going to 20 hp OB's they are engines that rev to about 6000 rpm and the reduction seems to be, usually, 2:1. This means there is a small fast reving prop that is far less effective than a 20 hp diesel.

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Old 24-03-2008, 19:54   #34
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For one thing, 20 outboard horsepower is more than 20 diesel horsepower. Outboards are measured at the prop, diesels at the crankshaft. You can lose 10-20% through the transmission. Some diesel gearboxes have little or no reduction - so the prop speed can end up being fairly similar. Then th epropellor itself also acts a s a reduction gear, depending on th epitch of it.
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Old 24-03-2008, 23:42   #35
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Not that I have looked at many normal boxes but the lombardini is 2:1 and the nanni 2 or 2.6:1. The same in saildrive is 2.2 for Lombardini and 2.5 for the Nanni. The Nanni claims HP at the prop and lombardini lists both crankshaft and prop with a 10% loss. Not that I have any expertise in this area but I would have assumed that the OB prop going at more than twice the revs would be less efficient. Do we have any prop gurus here.

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Old 25-03-2008, 00:50   #36
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A high speed prop is always less efficient , a prop turning at 1600 to 1800 rpm is a normal 3 bladed prop is 30 to 35 % efficient at 1000 to 1200 a prop is up to 50 % efficient , a prop on an outboard revving 2400 and more is only 25 % efficient.
This is one of the 2 reasons for me to choose slow revving props for the electric propulsion system.
If you take the 20 HP diesels you have in mind minus 10 % loss leaves 18 Hp at the propshaft - 65 % leaves 6.3 hp as propulsion power.
If you would use a racing folding prop you loose another 5 % in efficiency or 5.8 is left. All these percentages are not exact but jus to give an idea of the losses that occur between the crankshaft and the actual output.
With an electric motor of 10 Kw or 13.3 Hp out put at the shaft and low revving 3 bladed prop the actual propulsion output will be around 6.7 and the torque is there from o rpm so the effect and acceleration will be higher that with the 20 hp diesel.
On Green Motion we use props with a .71 blade area and the loss is around 46 % or even more efficient. these where designed for us and can only be used because they come out of the water when sailing otherwise the added resistance would be killing for the speed
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Old 25-03-2008, 01:29   #37
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I'm going to start a big disagreement here, but were I to have the option I would think about retractable yamaha 9.9 outboards off of inner cockpit and save myself 1000 lbs of weight.
schoonerdog... Just for your info... The Yamaha 15 Enduro is EXACTLY the same weight as the 9.9 and very little different in price. It is the same engine but with different jetting. I was going to buy a 9.9 for my dinghy but when I was told that I went for the 15 instead. After 5 years of 30 plus weeks a year bareboat charter that Yamaha still fires up easily and runs smoothly.
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Old 25-03-2008, 01:53   #38
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Thanks Gideon.
The difference is probably less than I would have anticipated but seems to support my gut feeling.
So your experience suggests that a 20hp diesel equates to about 13hp electric, if geared appropriately. That is close to the conclusion I came to when I was exploring the prospect of fitting electric. If I remember I concluded I would need a min of 7 and prefereably 9 Kw a side.
http://www.cruisersforum.com/forums/...earchid=226063

Spoke with the designers and they are quite happy, indeed quite enthusiastic, about the change of position. It would appear that this is the way I will proceed.

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