PDA

View Full Version : Blackhawk Propeller Performance



DonCig
07-13-2007, 06:56 AM
I would like to hear from some of you Blackhawk owners on what percetage of prop slip you are you seeing at your top speed runs. I was talking to Brett at BBlades and discussing my recent testing results with him.
It appears after my conversation with him that sometimes with almost all types of prop driven boats the highest speed obtained can be with an induced higher percentage of prop slip (usually by adding trim to the outdrive; more trim than you might normally add).
In my case, by overtrimming the drive and inducing excessive prop slip I gained 4 mph. The risk that ones takes when doing this is first you need enough rpm room with the engine prop combination to allow it; so if you are already on the prop limiter than you probably can not tell if this applies to you. You also need to make sure that the boat can still carry the bow; or else you will get a high speed porpise or an instant bow steering condition; both leading to some tense moments.
I talked to Gerometto and he has seen the same results on his boat; 16% slip to 19% slip yields his top mph in a slightly overtrimmed position.
In my case I saw my top speed increase when going from what felt right to an actual 17% slip.
In my case my Blackhawk drive is most effecient at a 12% slip rate; but this setting is not necessarily the fastest scenario.

Here is a link to a prop slip online calculator to assist you in posting your results.

http://www.go-fast.com/Prop_Slip_Calculator.htm

Thanks!

blackhawk
07-14-2007, 08:06 PM
I would like to hear from some of you Blackhawk owners on what percetage of prop slip you are you seeing at your top speed runs. I was talking to Brett at BBlades and discussing my recent testing results with him.
It appears after my conversation with him that sometimes with almost all types of prop driven boats the highest speed obtained can be with an induced higher percentage of prop slip (usually by adding trim to the outdrive; more trim than you might normally add).
In my case, by overtrimming the drive and inducing excessive prop slip I gained 4 mph. The risk that ones takes when doing this is first you need enough rpm room with the engine prop combination to allow it; so if you are already on the prop limiter than you probably can not tell if this applies to you. You also need to make sure that the boat can still carry the bow; or else you will get a high speed porpise or an instant bow steering condition; both leading to some tense moments.
I talked to Gerometto and he has seen the same results on his boat; 16% slip to 19% slip yields his top mph in a slightly overtrimmed position.
In my case I saw my top speed increase when going from what felt right to an actual 17% slip.
In my case my Blackhawk drive is most effecient at a 12% slip rate; but this setting is not necessarily the fastest scenario.
Here is a link to a prop slip online calculator to assist you in posting your results.
http://www.go-fast.com/Prop_Slip_Calculator.htm
Thanks!


IF I remember right my blackhawk ran 17-18% slip at max speed.

Just curious, what makes you think you are "overtrimmed"? Since the blackhawk is a surface piercing drive overtrimmed to me would be when you start losing speed and/or handling gets worse. I could trim my blackhawk to high and I would simply lose speed. Trimming for max speed te me equals the right trim angle, not overtrimming.

glashole
07-16-2007, 05:42 AM
I ran 11% slip yesterday

trim was almost at the 1/4 point and more seemed to lose speed :)


Shea

DonCig
07-16-2007, 07:46 AM
I ran 11% slip yesterday
trim was almost at the 1/4 point and more seemed to lose speed :)
Shea
Shea, were you up next to the rev limimter, if not, if you feel comfortable with this manuver; next time try this experiment if the boat will handle safely.
Try trimming your drive up to the point that it will still safely carry the bow, even though you can tell that the props are slipping and then take the boat all the way up to full throttle but not over the safe limit rpm for the engine. What we are trying to determine is if your fastest speed is obtained with more prop slip than you would normally think is best for top performance. In other words sometimes a little more prop slip can be bennificial if you have the engine rpm range to make it work. This is what I am looking for input on from Blackhawk drive owners.

Thanks,

Don