Technical Question

GTR · Jan 17, 2003

Hi & I guess this is directed at Jimboat,

Can anyone explain the relationship between prop centre of thrust, distance from transom & distance/height from static CofG. Don't want to get into centre of lift as Jimboat has already explained how much this moves depending on hull attitude.

I think I am looking for a better understanding of the effects of positioning of the prop in relation to the other 2 on handling. What positive and negatives have people witnessed.

In model sense I guess this can be addressed as drive dog to transon (sponsons) and transom inset (engine) V static CofG in relation to sponsons.

Grim, JD, Jimboat, anyone want a shot at this.

GT ???

Jerry Dunlap · Jan 17, 2003

Hi GT,

Well, I wish I could provide a technical explaination of the relationship between the distance from the transom to the prop, but I cannot.

Here's what I've found based on years to just messing around with prop distances from the transom.

1. A propeller too far away from the transom makes the boat difficult to trim.

2. A propeller too close to the transom can affect the top speed potential - but the boat will probably corner really well. However, that also depends on the design of the boat.

3. A "Happy Compromise??" - Here's what I'm now running on my 3.5 and 7.5 boats.

3.5 tunnels(Villain S1 & JD WOF 30) 4"&4 1/4"

7.5 JD WOF 34 - 4 5/8"

These are distances from the back edge of the sponsons to the back face of the drive dog.

Grim actually runs his motor further in on his Villain because he does not use an adjustable motor mount and I'm using a DuBro mount.

JD

GTR · Jan 17, 2003

Thanks JD,

Now if we can just get some of the others to provide their measurements we might have enough info on variations to have a shot at how the distasnce effects handling.

cheers

GT

Slideblues · Jan 17, 2003

Well I measure mine transom to drive dog, but if you factor in the transom inset mine are close to JD's

The figure I use is 5 1/4" Have also tried 5" and 5 1/2"

Gene ;D

Grim, what cha think?

Jimboat · Jan 17, 2003

The location and relative "force" of the propeller (at it's centre of thrust), it's distance from transom and it's height above the water surface will affect acceleration, handling and stability. It's relation to the static CofG is not really of any interest, exept that you may know where the static C of G is. This Center however, is not nearly as influential as the propeller thrust (equal to the total drag on the boat) or the total lift forces (equal to the deadweight of the boat.

The distance of the prop line above water surface affects the thrust "moment" around the dynamic C of G. When this dimension is bigger, the affect of the thrust is to move the dynamic C of G fore. Here's a picture of all forces interacting, from my STBD book.

Grimracer · Jan 19, 2003

Jimboat

I like you...You are my hero.. Your brain is way to full of information and we will have to keep an eye on you so you do not pop a vein... ;D Guys...keep an eye on Jimboat, he knows way to much and he could explode...lol

Jimboat.. you are a very valuable addition to the I waters gang and all the tunnel guys are very glad you stop by..

Jimboat and gang.

What I have found is that as the boat reaches critical speed the prop has a better chance of maintaining full thrust at a closer setting to the transom. This regarding pitch. I have seen boats with close setup come out of the corner so hard that passing is best in this area of the race course. We (most of us) do not have on the fly adjustable trim. Having said this i could see longer setups working better for me. For now it is a balance of time tuning on shore and test, test, test.

Grimracer

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