Prop selection

[njensen]

I know prop selection is best done with trial n' error. However trying to get it right the first time would be "fun".

I'm close to desiding to change my engine from a DT 65 to a E-tec 90. I need a max speed of no more than 32-36 knots. I need a lot of hole shot, but would like to keep some fuel economy also. I need to be able to get on the plane having +500kg gear in the boat. Top speed at heavy load should be around 28 knots. The current DT 65 does the job but it struggles a lot.

The data below shows pretty much real life for my DT 65. The Efficiency pct. is set to make the real speed correpsond to real engine revs. I guess 90% is not totally out of line.

if i put a E-tec 90 HP with a 17 inch prop i get a teoretical max speed of 34,6 knots.
Do you think this seems right ?

Do you think i would gain hole shot by doing this change (the E-tec can turn a 13 x/x prop) ?


------------------------------------------------------------------------------------------
[Engine] [Dia] [Pitch] [Eng.rev] [Gear] [Prop.rev] [Eff.] [Speed [knots]]
------------------------------------------------------------------------------------------
DT65 11 1/8 14 4000 1,92 2083 90% 21,6
DT65 11 1/8 14 5000 1,92 2604 90% 27,0
DT65 11 1/8 14 5900 1,92 3073 90% 31,9
------------------------------------------------------------------------------------------
[Engine] [Dia] [Pitch] [Eng.rev] [Gear] [Prop.rev] [Eff.] [Speed [knots]]
------------------------------------------------------------------------------------------
E-tec 90 13 x/x 17 4500 2 2250 90% 28,3
E-tec 90 13 x/x 17 5500 2 2750 90% 34,6

[Pikey Dave]

Try asking here http://www.etecownersgroup.com/

Also lots of other info Re e-tecs. Whatever you do make sure that you call it at E-TEC not etech or E-TECH or you'll have the cousins & the ex-cons jumping down your throat

[BogMonster]

I don't think your figures are too far wrong - I spent a fair amount of time looking at what figures people claim for speed @ rpm and 5-10% slip seems to be the right sort of range at high rpm though I guess slip increases with a heavier boat.

I second the recommendation for the owners group, some really good stuff on there

[Pikey Dave]

I'd be tempted to look at 4 bladers, they tend to hang on better in the rough & give you better holeshot, however, they aren't as fuel efficient as 3 bladers. Everythings a compromise & the perfect prop still has to be designed. BRP has a prop exchange program with certain dealers which allows you to try out different props, that would be worth a try. Any dealer worth his salt would work with you to find the right prop, I tried 3 sets before settling on my current props. It's unbelievable the difference just changing the props can make & well worth spending the time to get it right.

[willk]

Originally Posted by njensen

Do you think i would gain hole shot by doing this change (the E-tec can turn a 13 x/x prop) ?

Be aware that the 25" shaft E-tec 90 has (certainly used to have) a larger gearbox (4.3/4" gearcase) and can spin up to a 15" prop. This may effect your plans if you are fitting this type. I don't know what the standard transom SR5.4 requires...

[njensen]

I'm going sailing with the dealer after he mounts the engine. However we are only able to test the boat without too much load. The real deal will not happen until i put in 4 persons and 300kg diving equipment.

the 90 are having a 20" shaft and a 2:1 gear factor. I think it's capable of 13 1/2 " props.

I read somewhere that long shaft (20") are to be used with keel2transom(top) height of 50 cm. The extra long shaft (25") are used with keel2trasom(top) of 60 cm.

My boat is 55 cm... so just in between.. i have always used a long shaft and will stick to that.

[Pikey Dave]

make sure you can get at least 5500rpm wot with a light load, 5800 would be better, you need to get 5000 rpm wot minimum fully loaded.