Pacemaker Boats

[merwin10]

Thought I would pass on some ideas on prop calculations - First and formost is you need to know several things about your boat - First are the tachs accurate - Check them against a intrument that is accurate to within +/- 5 RPM - Next Speed is measured by a GPS SOG = Speed Over Ground - SOG should be taken by making four passes in each of the cardinal headings North East South West while waiting enough time to pass for the engines to come up to WOT with neutral steering - add them up and divide by 4 - This will average out any wind/current differences - Next you need to know your transmission reduction ratio - Then your pitch!

What you can do next is figure your Theoretical Speed against your SOG -

Here is how it works -

Example 4200 WOT - 1.91:1 TRamsmission Ratio - 15" pitch prop

Take your engine speed in RPM and Divide by the transmission ratio this will equal Shaft RPM (SRPM)

4200/1.91 = 2198.95 SRPM

Now take the Shaft RPM and multiple it by the pitch -

2198.95288 x 15 = 32984.29319 Forward inches per minute

Now mulitple that by 60 minutes in a hour -

32984.29319 x 60 = 1979057.592 inches forward in a hour

Now there are 72913.44 inches per knot so

1979057.592/72913.44 = 27.14256235 Knots Theoretical Speed (KTS)

Now you take the KTS Divide by SOG = 18 knots

SOG/KTS = Slip Factor (SF)

27.14256235/18 = 33.68349029 % SF -

Now most planing boats run between 15 to 25% slip factor the lower the better - What causes slip? Drag is the biggest factor anything hanging in the water or dirty hull - Prop Cavitation - The single bigest factor over looked on inboards is rudder alignment - The less drag and the more bite your prop can get on the water the less the slip factor will be -

One member changed his three bladed 18" pitch prop for a four bladed 15" pitch props and gained 800 RPM, which we would expect - whilst reducing his slip factor by 4% - yet 30% is still high and improvements can be made -

Slip factor is one tool that you can use to measure your improvement of your Correct Prop pitch - and in todays ecomany with higher fuel prices it will make a big diference if you slip factor is as low as possible - So check those hulls this winter and align those rudders while the boat is out of the water -

Hope this helps -

Mike -
_________________
"Best part of Boating -- is the people you meet"

[Tide the Knot]

Mike,
Thanks for the helpful information as I am still going through prop issues with my 36 SF.
I apoligize if this is a stupid question but by rudder alingment, are you referring to their relationship with the shafts, or the positioning of the other rudder.
Thanks,
Andy

[neverending]

Here's the prop calculator I used when re-gearing my 25 Alglas. The results were pretty close. The same site has Excel spreadsheets as well.

http://www.surfbaud.co.uk/wave/Propking.htm
_________________
Dennis Sherod

[merwin10]

Andy -

Rudder alignment I will try and answer in a new topic in this forum so that it won't get lost here -

Mike -
_________________
"Best part of Boating -- is the people you meet"