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Old 06-05-2011, 08:23 AM   #13
Nomad Willy
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City: Concrete Washington State
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 18,743
RE: Calculating hull speed.

There is definitely only one way to properly prop a boat. And there is a narrow range of proper power loading too. People buy big heavy old trawlers and unlike the owners that had the boat when it was new they are seriously lacking in money for fuel so so they resort to desperate measures to save enough fuel so they can operate their boat. That is the only defensible reason for underloading or "cruise propping". There is just no other possible explanation for it. If everybody here had a credit card that someone else was paying for there would be no underloading or overpropping. And the new boat buyers establish what the boat will be... what it will look like and how it will perform and everything else. For most of us we have hand me downs that we can't afford to use they were intended to be used. You don't see Walt or Mark over propping and underloading because they have plenty of money for fuel. If I had enough money I'd have a lobster yacht w a DD 6-71 or a Uniflite 36 w 2 6-71s. And I'd be running 18 or 20 knots. You don't match a prop to a boat for greater efficiency. You match an engine to a boat first ( hp actually) and then you run it as it should be run. But it's back to money again. Solving the underloading issue would be to repower w much less hp to go the speed you want/need to go but that takes lots of that money stuff again. And slow is'nt the speed you really want to go so if you had the money you'd be going faster anyway. So if you're desperate to save money on fuel you can't afford to repower and the only option is to limit time underway and reduce fuel burn as much as possible with what you've got. So you're right Fred, the "cruise prop" and underloading is a questionable but viable way to make an old trawler work but it's absolutely not the right way to run a boat and the're is a possibility the engine won't like it but it seems the're are some like the Ford Lehman that will take it. And yes there is a "best prop" and that is a prop that will allow the engine to be loaded at rated rpm and WOT. That is the best prop and anything else involves sacrifice and limitations that should'nt half to be made. And lastly you don't need extra speed "in a blow". You're just use to lots of extra power in your cars. Anybody with any sense slows down in a blow. But if you need "all out" fuel efficiency, with the right engine, and realistically realizing the limitations of "cruise propping" it can be done and it can save fuel****** ......... but it's NOT the right way. The're IS only one right way to power a boat.
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