Genesis prop shaft/strut height and angle

[JonD]

I am running a Genesis which I have fitted out with a Leopard 4082 1550Kv with M445 prop, 6S batteries, and T180 ESC. (My first Offshore Electric Racing boat, so I'm still very much on a steep learning curve). It seems to be going pretty well. I am getting 60 mph (97 kmph) and it seems quite stable at full speed -runs flat and level. But it does some porpoising while it is accelerating and also in turns - not real bad, but I'd like to eliminate it, before I move up a prop size to a M545. I also get quite a large rooster tail of spray, but I don't really know what to expect in this regard.

COG is at 0.33 I have the strut height set so that when the hull sits on a flat table, the bottom of the strut is also just sitting on the table, and it is angled slightly i.e. angled in the direction such that the prop end of the strut is slightly higher than the end where the flex cable goes in.

Seems to me that the 3 variables that may cause or prevent porpoising would be COG, strut height or propshaft angle. I am happy to play with these in turn and try them, but I would like to know what effect I could likely expect to see from changing each of them? I am guessing that there may be quite some interaction between them which could make it tricky to find the ideal settings for each one. So any pointers would be really appreciated.

Cheers,

Jon

[martin]

If your running a stock rudder in line with the prop then offset the rudder to the right, a friend has the same set up as you have & although it was running ok it did porpoise a bit flat out . It also didnt corner that great at high speed ( i know the Genisis isnt great in turns ), i told him to off set the rudder. Totally transformed the boat, much faster & running flat & in turns its a different boat now being able to do sharpe turns with no problems at all staying completally flat in turns. He had done all the usual moving cg & strut adjustments.

[sundog]

Adjust the strut so that it is level (not tilting up at the prop end). That will help control porpoising and keep the nose of the boat on the water. Adjust it a little more if necessary. Sounds like you have everything else about right. I believe an inline 'strudder' is faster on the water than an offset rudder, but you will sacrifice some handling characteristics as noted.

[JIM MARCUM]

Ditto that. Prop shaft should be level on a flat board, or slightly lower on the prop end. Try level first, if still porpoising, lowering the prop end will force the front end down. Being out of the direct propwash, an offset rudder will be more stable and have less drag than a strudder.

Sounds like you are going to have a very nice running cat - welcome to FE boats John.

WARNING: Building & running FE boats is addictive, and may lead to a need for speed and/or competive racing. JIM

[sundog]

Being out of the direct propwash, an offset rudder will be more stable and have less drag than a strudder.

Jim I agree an offset is more stable, but I have always believed a rudder in propwash had less drag than one in undesturbed water? Recently had a winded 'discussion' with Rumdog that ended in a stalemate. Have you tried both ways (on the same boat/same setup)?

[JIM MARCUM]

Dave, I have ran one of my Apparitions with a strudder & now have an offset on it. But motor changes have made this an apples & oranges comparison. My theory is that an offset rudder will have less drag due to the fact that it is cutting thru the water at the same speed as the hull. A strudder contends with the much higher water flow pushed into it by the prop. I.E. more drag. It's both the increased water speed and water pulsation created by the prop that makes a strudder rudder more sensitive in turns and less stable than an offset rudder.

It's an untested theory, but seems logical to me. Maybe someone out there has done a more scientific comparison? JIM

[martin]

Simply moving the rudder out of the prop trust cone increased the speed a fare bit on this Genesis not to mention the handling. It ran like a totally different boat all round.

[Checkmateguy01]

I don't mean to high jack the thread, but i'm interested to know the "sweet spot" for the strut hieght/angle too for the Genesis hull. I have building a FighterCat Daytona, which is the the Genesis hull, and even tho i haven't yet run mine i'd like to have the set-up pretty close to right when i get to the pond. I have "pre-set" the strut with the bottom of the strut just slightly below the bottom of the hull (transom end, last step).

[sundog]

Checkmateguy: Bottom of strut level with bottom of sponsons is a good place to start.

On which rudder is faster: To keep from hijacking this thread (sorry JonD) I'll start another on this subject. Seems there is quite a difference of opinion on this subject! Sdg.

[JonD]

To keep from hijacking this thread (sorry JonD) I'll start another on this subject.

No worries Sundog, it's a really interesting topic. I will await the views of others with interest!

I adjusted my strut angle as suggested. It's now level . Tried it on the lake today. It's reduced the porpoising, but still there over a fairly narrow speed range. I'll angle it a bit more and see how that goes, and then may try moving the C of G forward just a little. I think it's all pretty close now - just a case of fine tuning.

Tell you what though. This is certainly a lot of fun, and has the neighbors all quite enthralled. Yep, I am lucky enough to have a lake right outside the front of my house (and intend to make the most of it!).

Thanks for everyone’s assistance.

Cheers,

Jon

[nata2run]

Jon, where about in Aus?

[JIM MARCUM]

Jon, looks like you are doing the right stuff. If moving the CG forward a bit dosen't stop the porposing, try adjusting the prop shaft with the prop slightly lower - still only touching the bottom of the shaft. That will lift the stern & lower the bow and end the porposing. You may also try raising the shaft 1/32" - 1/8" above the surface - especially if you are running a strudder. Please keep us up to date on what you find works best. JIM

[Mattwarner]

My genesis has the strut set about 1/8 lower than the rear sponson is this too low? ( centre line of shaft in line with bottom of sponson ) what effect does raising or lowering have on the characteristics?

[JonD]

Jon, where about in Aus?

Sorry, long way from SA. I'm in Queenland - Sunshine Coast.

[nata2run]

Nice! Soooo many boating days over there. Very little here in SA