Need some CAD help?

[joefroooo]

Terry if you haven't figured it out yet go to your RH view and rotate the part 90 degree and it should show up correctly in your ISO view if I am understanding you correctly/

Joe

 

[Terry Keeley]

Terry if you haven't figured it out yet go to your RH view and rotate the part 90 degree and it should show up correctly in your ISO view if I am understanding you correctly/

Joe

Thanks, that's exactly what I'm trying to do. I can rotate the axis' (UCS - Universal Co-ordinate System) but the drawing of the tub doesn't rotate with it.

 

[Terry Keeley]

Sometimes it's just better to bite the bullet and start over.

Whatchathink Tyler, will I get any useful info putting this simple shape into CFD?

 

[RaceMechaniX]

Hi Terry,

I don't think you will learn too much. You can see what the drag ratio is. It should not have any lift since it looks symmetric. Unless you change the incidence angle. This I learned is quite important as my SAW riggers with the sqaure'ish cross sections need to run ass end down and front end up.

What may be useful is doing different iterations to see the changes. I ended up laser cutting 8 iterations with different angles, widths, kick-ups, etc in till I found a decent combo.

-Tyler

 

[Terry Keeley]

Hi Terry,

I don't think you will learn too much. You can see what the drag ratio is. It should not have any lift since it looks symmetric. Unless you change the incidence angle. This I learned is quite important as my SAW riggers with the sqaure'ish cross sections need to run ass end down and front end up.

What may be useful is doing different iterations to see the changes. I ended up laser cutting 8 iterations with different angles, widths, kick-ups, etc in till I found a decent combo.

-Tyler

Thanks, I'll give it a shot anyway, again I'm just interested in some changes to the rear.

It isn't quite symmetrical but close, should have more lift on the bottom, especially since the air on top is all messed up with the cowl/pipe etc.

Surely your "squarish" boats weren't running tail down on the water? If they did they'd blow off as the speed increased, no? Do you think they mighta come up on the prop at running speed to make the hull run nose down?

 

[RaceMechaniX]

Terry,

My most recent hulls have a flat bottom for the forward 2/3's and I have a kick-up on the bottom for the last 1/3.

The top is a combination of arc's, so it does resemble an aerofoil on the top. Even with the aerofoil tub shape, most of the lift comes from the sponsons.

I found it better to not try and lift too much in the tub, but control the lift with the ride pads and angle of the ride pads. This made the boat less sensitive as speed although they are stickier compared to the fully profiled tubs.

Even with the profile and kick-up, I found my tubs and set-up ran the best with the strut raised up resulting in the entire tub slightly inclined. This has seemed to help keep the tail planted in the water instead of blowing the tail.

Tyler

 

[Terry Keeley]

Terry,

My most recent hulls have a flat bottom for the forward 2/3's and I have a kick-up on the bottom for the last 1/3.

The top is a combination of arc's, so it does resemble an aerofoil on the top. Even with the aerofoil tub shape, most of the lift comes from the sponsons.

I found it better to not try and lift too much in the tub, but control the lift with the ride pads and angle of the ride pads. This made the boat less sensitive as speed although they are stickier compared to the fully profiled tubs.

Even with the profile and kick-up, I found my tubs and set-up ran the best with the strut raised up resulting in the entire tub slightly inclined. This has seemed to help keep the tail planted in the water instead of blowing the tail.

Tyler

Interesting. Sounds like you've been fighting keeping the prop hooked up too, that's why I'm trying to have a better look at that rear rocker shape. That and moving more weight back will help I'm sure.

Only 8 weeks 'til the water gets soft here again, I got some work ta do...

 

[RaceMechaniX]

Terry,

I have had a lot of chats with the experts and most agree I needed more tail weight than I thought. I weighed all my boats, found the CG, measured the after-plane and measured the tail weight. The boats with more tail weight regardless of the after-plane or position of the CG ran better with less hoping.

I am basing most of this on my electric riggers, but the same likely holds true to nitro and gas SAW riggers.

I took one boat and make forward extensions to move the rear boom to where the front was and move the front way ahead. This looks like most of the European riggers with the side plates extending forward. This added a pound of tail weight on an 8 lb boat and made a huge difference. It's easy to do this for an FE hydro where we can launch any prop, it may not be so easy with nitro and gas trying to get off the beach.

Now, I have seen Mark Grim's 80 hydro run 130+ and that is a short little boat and it very very sensitive to the prop. Mark tends to work on the props to get the right ride attitude.

Martin on the other hand has a loooong Cadillac for a 80/90 SAW hydro and it runs almost as fast. I don't think I have seen Martin ever blow the tail on his SAW boats. He has a very long after-plane like I do and still manages to get his boat off the beach with relative ease. Pretty sure both guys run rear shoes.

-Tyler

 

[Terry Keeley]

Thanks Ty, sounds like we have both been down the same road.

I ran a couple of winglets on my first SAW boat that was way to short and way too light on the prop, they helped a bunch to keep the prop hooked up. They of course added a bunch of drag with the down force where a "diffuser" is supposed to be nearly drag free.

My latest boat has a 29 1/4" afterplane and balances at 9" back or about 31%, prop weight is 1lb 14 oz. I added 6 oz to the bait box moving the CG back 3/4" to 33% and made the prop weight 2lb 3oz., it helped a bunch. I'm thinking a prop weight of 2 lb. 12oz or so plus the aero of a diffuser will be a good balance to keep these high pitch props I've been working on hooked up.

Remember what tail weight your 60/80 boat had? PM me if you want.

 

[shoboat]

Terry,

The higher rake angled propellers won’t blow out the tail of the boat.

This is another reason why Martin is doing so well with the new ABC propellers.

I know Mark Grim is using the newer ABC propellers as well. Jim has almost all

the popular sizes done now and still working on more. I know you like to make

everything on your boats, but this is one area that Jim has just about perfected.

Spring is coming soon I hope?

Good Luck On The New Designs,

Mark Sholund

 

[LohringMiller]

Mike Bontoft and I did a lot of ground effects aerodynamic testing on our gas riggers. The findings are summarized in my High Speed Boat Design Part 2 in the April 2017 NAMBA Propwash. Stability is more important than low drag. Air drag is insignificant compared to water drag. We found that building a base tub and trying various sponson and air dam combinations gave a lot of information. The basic principle was that forward sponsons stabilize the tub's lift. Joerg's rigger below applies all the principles of a stable design using a combination of wings in ground effect.

Lohring Miller

 

[Terry Keeley]

Here's what I'm trying to do, cool stuff:

 

[Terry Keeley]

Moved...