Hello, I'm trying to "maximize" my boat as much as I can until I can repower. Currently have the original Cummins 555VT-M 270hp engines. The are working well. I bought the boat with the current props. Last time I had them checked they were 22x22 4 blades. Seems to me the boat is slow for the power. At 2300rpms I'm seeing 13.5knots and at 2550rpms I'm seeing 15.5knots SOW. I would guess I'm missing around two knots from my mentioned speeds. This was done on a clean hull and around 3/4 full tanks. I found the original owner had some 3 blades stowed away which seem to be the original props. No tsure on the specs but I'll have them inspected. Any input on props you've guys have used with the original diesels? Maybe some speed numbers so I can compare? If I do prop slip calculations, I'm at 49% which seems high.
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Edited: Apr 22
1976 Chris Craft 36' Commander SF Props (Cummins VT555-M)
1976 Chris Craft 36' Commander SF Props (Cummins VT555-M)
6 comments
Comments (6)
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Is 2550 your max RPM? The VT555 WOT is rated at 3000RPM. If that is all you are reaching you either have engine performance issues or are overpropped. I suggest you review the performance with a prop shop first. Our '76 47' has VT370's and I can only get to around 2850. At that RPM I can reach 16-17 knots. I have new Acme four blades and I've given up trying to reach 3000. If I have to I'll cruise at 2600 but that is seldom and not worth the cost and stress on old engines that still run well and replacements are not in the budget. From what I know the 555, 903 and 370's are all very similar in design. Good hunting.
Thanks for your input. I understand that trying to reach 3000 RPMS would be a good test to verify a good prop match. I have not tested them in that scenario. I'll give the 3 blade a try after I have them measured and rectified if need. From what I read 3 blade usually give a faster cruise and top speed at the expense of some vibration.
AS Michael indicated, it all starts with max RPM's compared with what they are rated at. If you can't get there, you either have a performance issue, too big of a wheel or a rough bottom. Not all clean bottoms are smooth. This creates drag and increases slip. Your slip number seems high, but, it could be caused by several issues. Too large of wheel is one of them. I would start with WOT and also find out what factory wheels were. If you get the build card from the Mariners Museum it will have prop numbers on it. You can then look in the CCCC archives for the CC prop # chart and find out what diameter and pitch they were. Good luck. I hate prop issues because there are so many variables and few silver bullets.
Agreed.
Be sure to photo-tach the engines first so you can account for any delta's with the tachs.
The tachs have been verified already. Thanks for the suggestion