MODIFYING AN OFFSHORE BRACKET --GREAT RESULTS!

[Sharphooks]

Thought I'd do a brief thread on a recent modification I had done to an offshore bracket.

Boat---24 foot Skagit Orca
Power: Suzuki DF300

The depth of the original bracket was approx. 24" with approx 36" set-back. Mounted to the bracket: the DF300 with a 30" Leg

Because the motor sat so deep in the water I was experiencing huge prop slip and lots of water commotion behind the boat at any speed other then WOT (soaked the cowling continually at cruise speed)

In response to an earlier post about this problem, there were recommendations to mount a jack plate to the existing bracket.
I considered that but finally settled on modifying the existing bracket so it extended to the bottom of the hull

Here's the original bracket:



Here's a pic of the modified bracket---I had hoped this approach would add floatation under the DF300 and allow the ventilation plate to be properly placed in relation to the bottom of the hull...yes, I also splurged on a Honda BF15---That Suzuki 9.9 was sufficient but the design was stupid---no way to flush the salt out with ear muffs unless I placed duct tape over the water intake ports (???!!)--what were they thinking?



Spent a pile of hours preparing for painting---coat of zinc chlorate etching paint, then multiple coats of anti-fouling paint (the boat is wet-slipped so that was the necessary evil )



Final paint job before launching her:

 

[Sharphooks]

I purchased a PowerTech OSF4 16" diameter stainless 4 blade before the bracket mod. AS noted, I was getting HUGE prop slip and it looked like I was dragging a bundle of logs behind the boat ---lots of water turbulence and commotion when trying to get up on step.

Here's a pic of what the wake looks like post-bracket mod at 4,000 RPM--- before the mod you would have seen rooster tails of water coming up over the transom, soaking the DF300 cowling:

 

[noluck]

looks great now!

 

[Sharphooks]

I thought I'd post the "before and after" prop slip calculations

Once again:

Boat: 24 foot Skagit Orca---5,500 lbs with 150 gal gas (heavy!)
Power: 2013 DF300
Prop:: POwerTech OSF4 16" diameter 17P

Before Bracket Modification After Bracket Modification
RPM MPH GPH MPH GPH
3200 9.9 6.7 15.1 6.9
3400 10.8 7.8 17.0 7.9
4000 16.6 10.5 23.5 9.7
4200 20.4 11.4 24.8 12.1
4300 23.2 12.3 26.1 13.8
4600 25.2 13.2 25.1 13.2
5500 31 20.9 32 23.0
5800 33.7 26.6
6000 34.0 25.0
So before the Mod, WOT was 5800, 33 mph , fuel burn approx. 100 L per hour After the Mod, WOT was 6,000 MPH = 34 and change , fuel burn was approx. 94 L per hour

What is really nice to see: check out the increased speed relative to RPM from 3,200 up to 4200 in the "pre-mod" and "post-mod" results---pretty dramatic, although once you get into the 5's, the numbers are more or less similar

It would be interesting to move up to an 18P or 19P propeller to see what kind of speed I could milk out of that DF300 but this is a HEAVY boat with lots of fuel capacity . I plan on doing a 1,000 nm trip this summer. My goal was to bring fuel burn down at cruise (3,700 - 4,000) and target "reasonable" speed which I think was achieved

Plus, the boat , with its 20 degree dead rise has a VERY tender ride---you continually have to fine tune the tabs as you ramp up through the RPM's, especially once you cross the 4,500 RPM threshold---kind of a white knuckle deal that I don't have to deal with at cruise speed.

Yes, I could have gone the jack plate route to get that 30" leg out of the water but Increasing the floatation beneath the DF300 seemed to be what she needed the most----at this point, I'm glad the boat was hung with a 30" leg (I'm the second owner) --- it seems to complement the bracket modification very well and will pay off when running in following seas

 

[bigdogeh]

impressive numbers, especially when you compare the before and after. seems like you made a great decision extending the transom depth. looks like the welder did a nice job also!
thxs for sharing!

 

[gungadin]

Looks like you shaped the extension to help deflect the water outward as well. With the shape before the mod, do you think the squared off back was contributing to the spraying of the cowl?

 

[Sharphooks]

This is guesswork but I think the real issue was how the water was flowing once it passed by the aft-most edge of the keel (prior to the bracket mod.)

I'm thinking that there was laminar flow up until that point then once past the transom, it became a jumbled mess of "dirty" water under the old bracket. By the time the prop bit into it, it was already mixed with air from all that turbulence. Not enough turbulence to promote cavitation but certainly enough to dramatically effect prop efficiency and prop bite

By extending the keel of the boat aft (which is what that bracket mod is actually doing), I'm thinking it extended the flow of laminar water back towards the prop an additional meter ---the result---better bite/less turbulence/more efficiency

And yes, I'm sure the design itself helps to deflect water and smooth things out

I should of taken a picture of what my wake looked like before the bracket mod---it literally looked like I was dragging a dead cow back there. Now it's smooth as silk

 

[Wild Bill]

I'm thinking about mine... So did you continue the hull at the bottom? Is it a bit of a step? Still have a drain plug?

 

[Sharphooks]

The add-on bracket was welded flush with the keel/transom and extends aft more or less in a straight line with the keel (when I get her back on a trailer I'll confirm if there's any rise in the keel line of the add-on ----if there is, it's subtle, perhaps 5 degrees)

There is a channel welded into the bottom edge of the bracket add-on which allows access to the drain plug. It's best pulled with a socket wrench and an extender but in a pinch, there'd be enough room to use a crescent wrench if you were on your back

The add-on has its own drain plug---hope I never see any salt water coming out of that hole....

 

[Rayvon]

My boat has the same pod style yours had (doesn't go down flush with hull keel) and when I bought it last year it didn't perform as well as I thought it should.I found out that the motor had been mounted with the cavitation plate level with the hull bottom.I did some research on line and found out that the motor should be mounted an inch up for every 12 inches back the pod extends.I raised the motor 2 inches and now it performs fine.I could have done what you came up with and achieved the same results and raised my scupper height at the same time,another plus,but my pod is fiberglass and it would be a bigger job to add on to the bottom.If and when I go to a four stroke motor(another 100 lbs. approx.) I may have to pull the pod off and extend the bottom as you've done. Orca's are really well built boats,glad you tweeked it to be even better.