Prop Question

[neutralbouyancy]

We are running 1992 290 Open with minimal which we just reproped with Michigan Wheel L/R 17DQX Nimbral props which are 4 blade rather than the original three blade as recommended by local propeller company after running a complete set of specs through their computer.

On 19" 3 blade props top speed was 25 knots with common cruise being 22 knots at 3300 RPM with new props under a lighter load only making 14.5 knots at 3300 RPM.

The boat runs Marine power gas 5.7 (350) EFI with throttle bodies, transmission from hearth marine ratio A 2.03 ratio B 2.03 model HSW450A-2.0

The propellar company was hgihly recommended and has done great work for others.

Is this performance comparable to other owners performance? Can someone provide knots with corresponding RPM for either a 3 or 4 blade configuration.

[Almarktool]

I'm confused,

you repropped the boat and went from 22knots at cruise down to 14.5 knots at cruise ?
so things got alot worse,

Mark

[neutralbouyancy]

Yes, that's our concern. We removed the old props, took photos with measurements and all the specs to the shop, then they ran everything though their computer and came back with a recommendation which we agreed too. We installed them last week and took her out on Friday and Saturday. Much to our surprise the perfromance was aweful. We went back to the prop shop today and explained the situation, they are re-running everything now.

I am just wondering what others are getting with a similar configuration?

[George L]

I have a 96 290 open and these are my numbers by GPS.

3800 rpm 23knots
4700 rpm 27knots max speed

Starboad motor has 1100hrs

Port motor is new 70hrs on short block. I used all the old marine power parts
except for manifolds.

I replaced the props in May. 3 blade made by high torque. They are stock size. After a lot of research I read that 4 blade props only help for planning faster, then the extra blade creates drag and slows the boat down on the upper end.

[Almarktool]

George,

what do you burn per hr ? I am getting a burn of 32gph at 4000 rpm at 23knots for both engines 5.7L captains choice 2007 luhrs 28 open

[neutralbouyancy]

After meeting with the prop gusy today they are making us new 4 blade 18's with 18 pitch which according to thier calculations should bring the speed up to the mid twenties for cruise and improve planing as well. We will put those on an try her out later this week. I will let everyone know how that works out.

Thanks for the feedback

[George L]

George,

what do you burn per hr ? I am getting a burn of 32gph at 4000 rpm at 23knots for both engines 5.7L captains choice 2007 luhrs 28 open


I have not had a chance to get a real accurate measurement but think at 4000 rpm I'm around 33-34 gph.

The B & G flow meter was remove by the previous owner and I can't find a replacement.

I did find something interesting though. The port motor (new vortec short block) burns about 10 to 20 gallons more every fill up.

I'm running conventional 15w40 in the port for break in and 15w50 Mobil 1 synthetic in the starboard (1100 hrs) The port motor oil runs at 230 degrees F at 4000 rpm and the starboard runs at 210 degrees F at same speed.
This is a direct benefit of synthetic oil, It typical runs 20 degrees cooler. I don't know if this is causing my increased consumption or if it's due to break in. Although at 70 hrs the motor should be seated by now with very little blow by. Also when I run the boat at wide open throttle for more that 10 minutes the port pressure begins to drop to 30psi. I believe the coventional oil is breaking down and frothing in the pan.

I'm going to change the oil soon and follow up.

[skippr]

Removing diameter is usually not what you want to do. I now see you went from a 19" to a 17" and lost performance. Diameter is very important when you load the boat with people and gear. A smaller diameter prop screws a smaller hole through the water which is affected by conditions placed on the vessel

For example pitch pushes you through the water and diameter keeps you there under adverse conditions either seaway or load.

[neutralbouyancy]

New the boats came with 18's and when the boat was repowered the previous owner increased to 19's with the intention of gaining speed and power. However 19's were damaged and also push to close to the hull whichis why we replaced them

After re-running the numbers they swapped out our 17's and we put on 18 x18 four blades yesterday and she ran great. Much more stern bite and up on plane easily cruising at 22 knots and a very smoth ride. This was definately the right way to go. The prop guys also cut down and repitched the old three blades for us so we will have a spare set as well.

Thanks to everyonefor their help!

[skippr]

What is your WOT rpm with the new props?

[CrazyIslandMoFo]

Well, I have had the issue with the uneven fuel burn... the motors shouldn't be running more than 170* deg. salt will start to crystalize in the intake. I found a kinked coolent hose to the starboard intake, the computers will try to "cuench" the cylinders w/ fuel. My luhrs (T290) will only cruse about 18 knots and will pitch badly when I try to run open throttle.(any one else notice this??) I have figured about 18+ GPH being hopeful... each inch of diamater will affect the RPM about 20x and is usally based on what will fit reasonably under your hull,... each inch of pitch will affect the RPM by about 200x and should be adjusted to suit your motors max RPM at WOT...at 4400-4600, (5.7 EFI) the Silver Star will run about 24kts.Four blades tend to be smoother and beter for pushing heavy boats while three blades run more efficient Also with out sea strainers one should check the raw water intake every so often when the temps are coming up, last season I had shell fish enter through the screens and mature in the hoses...:blink:. NEW risers and elbows also helped well, my temps are commonly at 150* and 165* when running strong. I have 1pair of recond 17X17 3blade for 1 1/4 shaft for sale not used since the recon dyno balance.