[Rhodes22-list] Props

[Michael Meltzer]

I am not expert, but the little I know:

Water has about 20 time the density of air. It is not compressible. A prop will rotate in water "pushing" the water creating a 
forward force. Lets talk in term of a wood screw, a screw has two main composite, It diameter and the pitch of the treads, a #6 dry 
wall screw with coerce threads is easy to drive because it is thin and goes in fast because it has a high pitch on the treads, OTHO 
A 1/2 lag bolt with fine tread is very hard to turn and will go in very slow. Props are the same way. The greater diameter on a prop 
the more force it takes to turn it and the more pitches it has the faster it will go. Because water can more "around" a prop their 
is also the term slippage, if you have  a 10x12 prop that mean it 10 inches diameter and for every turn it should move the boat 12 
inches. Well with low speed boats the slippage can be 50% or more. A little motor theory here. The motor need to be able to cover a 
whiled speed rang(idle to 5000+), the prop need to utilize the power buy still permit it to reach it full rpm range(to get full hp). 
Too large a prop and the motor will never reach full hp, to small a prop and your burn out the motor from over revving. The motor 
must produce more hp than is needed by the prop until it reaches wide open thotle, when the prop uses all the motor power it will 
limit it from spining any faster. The motor produce a "power band" that the prop must stay inside of. One of the factors is how fast 
the boat itself will travel(we get back to that). Next the force to turn the prop comes from the motor and the lower gearing, 
gearing of 2;1 for most "dingy motors" and 3:1 for most "high thrust", To keep the slippage down (and get more of the power in the 
water for a sailboat) and to have the prop trying to user closer to the produced power of the motor they try to use a larger 
diameter  slow turning prop (my old Yamaha high thrust prop 11 3/4 x 9 1/2, with 4 big blades). The Nissan 9.8 is using gear of 
2.08:1 and stock 8.5x9.5 prop, this is great for a diggiy going 15+ knots but with a slow sailboat the 9.5 pitch will not permit the 
motor to reach full rpm(or hp), it outside the power band. So you want to reduce the pitch(generally this is done based on the motor 
makers tables, their is still "black art" in place based on blade shape), even so this setup will have higher slippage that a high 
thrust setup. Generally you want the Biggest, lowest pitch prop, in the rhodes 22 case I found it better to back off one size in 
pitch otherwise you feel stuck in "second gear" all day.

FWIW I have a 15x13 on the new boat, Does that help, I does better with and more exact question, Rodger makes a better teacher :-)

MJM


----- Original Message ----- 
From: "Bill Effros" <bill at effros.com>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Saturday, September 11, 2004 9:59 PM
Subject: [Rhodes22-list] Props


Michael,

Talk dirty to me.  Explain props.  I understand the terminology, but I can't make sense of the practical significance.

Bill Effros


----- Original Message ----- 
From: Michael Meltzer
To: The Rhodes 22 mail list
Sent: Saturday, September 11, 2004 9:21 PM
Subject: Re: [Rhodes22-list] Outboards


I do not like one cylinder motors, to get 6hp out of it you might be geting a vibation problem. I would move up to the next size.
looks like the the 8hp is a derated 9.8, So maybe the 9.8 is the best bet, BTW make sure to get the 8.5x7 prop.

MJM


----- Original Message ----- 
From: "Ronald Lipton" <rlipton at earthlink.net>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Saturday, September 11, 2004 6:58 PM
Subject: [Rhodes22-list] Outboards


> My Merc. 9.9 outboard is dead at my mooring in Monroe Harbor.  I am thinking
> of replacing it with a new Nissan 4 stroke 6 hp.  It is especially appealing because
> it weighs only 55 lbs and I am going to have to swap motors on the water.
> (I still have to get the old behemoth out.)
> Does anyone have experience with these engines?  I know that 6 is a bit on
> the low side for the Rhodes but I tend to use the engine only close to and inside
> the harbor.  I also have heard that the four strokes tend to have a bit more
> thrust per rated hp. Clean and quiet is nice too. Any comments?
>
> Ron
>
> __________________________________________________
> Use Rhodes22-list at rhodes22.org, Help? www.rhodes22.org/list
>

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