16ft Boesch Design Restoration

[JamesW]

Adjustable cave plates, porpoiseing & props;

I started out with the normal cav plate that the rudder runs through, this already had timber wedges to adjust the angle and initially I made some with more angle to drop the bow.

I then made adjustable trim tabs, these worked well but as with the original cav plate they were creating a lot of drag to keep the bow down. Being able to adjust whilst driving is a great asset for rough water and wake crossings.

After reading about how some props lift the bow more than others I went and played with props, pitch will make little difference to bow lift, unless it makes the boat slower! The Rake of the prop is the main force in bow lift which as speed increases produces the porpoiseing, see explaination of rake here; http://www.propline.com/Propeller-Gener ... nology.htm . My current prop is a lundberg with obvious rake towards the bow, my Austral ski prop matches rpm and speed but needs heaps of trim, no trim for the Lundberg until I start to approach 50mph, then pull a little trim and all is good!

Trim tabs


Lundberg prop


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Uploaded with ImageShack.us
James

Chris, my props are 1" SAE taper (your's may be morse taper) you are welcome to give them a run and see how they go!

[viteau]

James, my prop shaft is 7/8" so thanks for your offer for others to try. Do you know what rake your lundberg has?. I am not after race pace but would just like to be able be able to safely use the full range of rpm without the bounce. My cav plate does not have a great deal of flex for adjusters and not sure whether they would make solve the problem without replacing the whole set up. Cheers Chris

[echotoo]

Prop comparison, Hijack ;

Austral 12HRSJ re-pitched to 12IRSJ H is 13" pitch, I is 14" pitch.
Lundberg RC12x15SCJ






Chris, I have a 7/8" to 1" adaptor sleve as my old shaft was 7/8" and 1" props are easy to come by, if you would like to give them a go!

Cheers,
James

[Damo]

Hey guys , my prop is also 7/8 like Chris's. The designation on it is 11GRSX, I know it is 11" in diameter with quite a pronounced lip on the drive side, could someone please help with what GRSX means?

[viteau]

Thanks James for the comparison photos. The rake on the Lundberg can you notice any loss of pulling power? I might have to come and visit your dam for a trial day. Cheers chris

[JamesW]

Damo, dislexia tells me you have a GSXR 11(00) !

11" diameter, G should be 12" pitch (since my H is a 13 and I 14), R for right hand rotation and SX should describe the cup. I thaught Austral had a cup key chart on their site but can't find it at the moment. http://www.australpropeller.com.au/

I see they make a prop like the Lundberg, http://www.australpropeller.com.au/cleaver.htm

Chris, Both props run at the same speed/revs despite the extra inch pitch, this seems to be due to less blade area on the Lundberg. There is very little diference in pull but a huge change in how the bow is carried.

Prop trial day at Glenbawn dam would be good, dragged out the 6 props I have and one that I have never run looks interesting, then another was used as a rotary hoe for a couple of years whilst attached to a boat is also interesting in a completely different way !

James

[Bob Vic]

Props - they are a black science. I've tried a heap of props on both the skiff and the Lewis runabout. Trial and error have been the only way to work through the maze. In terms of the runabout.... That three blader which has cleaver style straight trailing edges is what came with Turbo Fire (and the past owner reckon it was great) didn't do it for me post restoration. It buried the nose. Others that I tried torqued the hull over to one side. I finished up with a fairly conventional looking prop (don'tr recall what it is) but it works. Good engine revs and good handling.

The skiff - probably tried six or seven props and settled on the prop which lifts the bow a bit and gives me decent engine rpm. The hull doesn't handle on flat water, sucks on. I'm currently going smaller on the gears so will need to go through the prop testing exercise again.

So in short, grab a box of props, a prop remover, a shifter & a big hammer and try them one by one to see what they do for your hull.

Bob

[Damo]

Hey James, thanks for the feedback as its all great stuff even the dislexia diagnosis, allthough on this occasion i was still seeing straight allthough you may want to be checked, as per the pic mate it is definately 11GRSX , not that I imagine this changed the spec of the prop at all. again , thanks James

Current Boat Status, on the weekend i created a concoction of epoxy and sawdust and filled all of the bottom screw holes. Now i am boodly sanding again , once sanded i am going to prime the water line (Please feel free to supply suggestions )and varnish the side panels. I am thinking of then giving te whole lot a 1200 rub and spraying with automotive clear ( sorry did I say Automotive) . This will make the blending of the paint and varnish heaps easier, ( Please feel free to supply suggestions )

Love the feedback guys, Thakyou and please keep it coming!!!!

[screwit]

Auto clear will not stay on a car in the weather , most resprays peel off in time.
Under water line may not be so good and varnish is like enamel, so car paints dont match all that well and timber paint needs to flex and car paint dont.
Ross

[hookster]

Howdy Damo, is there any way that you could mount that cav plate onto your boat (when it is finished) with some sort of temporary adjusters in place of the fixed brackets you intend to use? That way when you are tuning the trim of the boat while experimenting with various props, you could finely tune the trim by adjusting the cav plate, and when you have it where you want it, then weld on your brackets at that angle and remove the temporary adjusters. Or if that is not feasible, make a second identical (but less fancy) cav plate, tune your boat with that one, then remove it and set the bling one to the same angle. That is a less than ideal option, because you will find that there are only millimeters of cav plate adjustment required to make huge differences in boat behaviour, and by removing the cav plate and putting the other one back on, there is no guarantee you'll get it in the same position and at the same angle. Just a few thoughts for ya.

I can't blame you for mixing up the G's, S's X's and R's on the prop you have. I had two GSXR11's in the 90's and they were a damn fine machine in those days, but probably quite agricultural by today's standards Jon.