need help with prop v rpm issue

[67hat34c]

we have a 34c with crusader 454's. 1.88/1 ratio. prior props were 3 blade, i believe when they were in good shape we got 4400 rpms wot, approx 27-28kts.

we just had bottom sanded to gelcoat and changed to 4 blade props, wot is now 4000. however speed is way faster. at 3250 we are at 23.8 kts, that is up from 18kts to 19kts. wot speed is 30+kts. @4000.

not sure if it is engines or the props causing wot to be lower. before we hauled her all we were getting is 4000rpms with damaged prop, bent blade not severe.

what do you think.

 

[doc g]

What rpm are the Crusaders rated at? Whatever rpm they are rated at they should be able to spin up. The only thing that could change the rpm 10% (all else equal) is a change in the props themselves . You have smoothed the bottom ,so if anything she should get more rpm not less at WOT. It has to be a prop issue assuming the engines are running correctly. The new props are bigger either in diameter or pitch or both.

 

[Nonchalant1]

It's the props. You're overloading your engines so they can't reach rated RPMs. Maybe the new 4 bladed has less slip than your old 3 bladed also.
What was your old dia. X pitch?
What is your new dia. X pitch?
Try taking an inch out of the pitch (a good prop shop can usually take up to 2" out). Before you do, some prop shops may have the same dia. 4 blade props with less pitch that they can loan you to try until you find out what you need.

Doug Shuman

 

[67hat34c]

engines are rated for 44-4600 rpm WOT. we just had these props done at a shop. we used them before but they were turning even lower rpm's they cut half inch off Dia. and put in a cup. I dont know what the pitch is but diameter is now 19" I think.

Will this slight overload cause engine harm? we typicaly run 2800 to 3000.

 

[Nonchalant1]

If you run it at 2800-3000 now, do the engine temps stay OK? You'll probably run higher exhaust temps but the same engine cooling water temps. Maybe Karl can tell you if your gas engines with solid blocks are less of a problem than wet liner diesels for overloading and temps.

Doug

 

[doc g]

If you tell the prop shop what its turning now and what you want they should be able to deliver. It sounds that you are 10-15% overpropped ,when the bottom is dirty, full of fuel ,water, and people it will be more over loaded. I would re-do the props again. They should have been able to take you from the 3 B to 4 B and keep the rpm where it should have been...i.e. 4400 rpm.

 

[UNIQUE_NAME]

assuming the tachs are accurate and engines/linkage are working properly, overloading the engines will only cause pain in the future and 400 - 600 rpm's in not really slight. you really need to get the engines at least to the lower wot limit (4400). running at 3000 rpms on a motor with a top end of 4000 is like running at 3300 on 4400 top end motor. (the 3250 would be like 3500) this could help explain your performance jump. a vauum gauge would confirm you are pulling a lower vacuum at your current 3000 rpm's than you were before.

 

[JLR]

Just changed my props today from 19x19 light cup (3 blades) to 19x16.5 cupped with (4 blades). Here are the results. Old props top end 4200. New props top end 4400. Old props 3600 rpm equals 21 knots. New Props 3600 rpms equals 20 knots but much lower gph on flo scans. New props turn up 22 knots at same gph as old props turning 21 knots. Not sure if that helps but it is one hour old info.