Repropping my cabin cruiser.

[rs2k]

Re: Repropping my cabin cruiser.

Maybe this is a better way to ask for what I need:

As fun as it is, I don't need to be cruising around at 40 MPH. Most time will be spent trolling at around 10 - 15 MPH which means the boat will be spending the majority of its time sitting deep in the hole. At the top end of my trolling speed the boat is just about to plane. What prop would be best suited for this type of work?

How would that prop handle at full throttle and at cruising speeds?

I understand that static thrust is more of a function of diameter than it is of pitch and that dynamic thrust is more of a function of pitch than it is of diameter.



As far as the props I have now, I have no idea if anything has been done to them, but I doubt it. This has always been a Kansas boat. There aren't a lot of prop shops in Kansas. Would taking a picture of the props I have help? I do know that these props have done some dredging in their time. There's no paint left on the blades and the previous owner told me they got the boat stuck more than once in our shallow lakes.

 

[rs2k]

Re: Repropping my cabin cruiser.

Some more info:

The spark plugs look a bit rounded from age and also show the engine is running too rich. This makes sense because the engine is most likely lugging.

For the following test I did a dry compression check. I did not use any oil. The service manual says the compression should be 135 PSI for the 9:1 compression ratio 260 HP Chevy 350. All cylinders read between 142 and 148 save for cylinders 3 and 5 on the starboard engine which read 132 and 135. The starboard engine runs and sounds superb, I do not suspect a head or cylinder problem between those cylinders. I suspect a bit of carbon buildup on the valves. The port engine had it on the cylinders 3,4,5 and 6 when I removed the heads for head gasket replacement. I will run some sea foam through that engine to see if it can clean up those numbers.

 

[rs2k]

Re: Repropping my cabin cruiser.

I don't know anything about prop brands, but how about one of these three?

http://boatpropellers.iboats.com/OM.../6508/?**********=505456989&*******=961584571

http://boatpropellers.iboats.com/OM...19910/?**********=505456989&*******=961584571

http://boatpropellers.iboats.com/OM.../1370/?**********=505456989&*******=961584571

 

[hwsiii]

Re: Repropping my cabin cruiser.

If you are going with aluminum props I would recommend going to 19" props because they do not have the same blade geometry of stainless props and I don't know of any that have cup built into them. My analysis is that with the cup you say you have in the SS props that usually adds about 1" of theoretical pitch to a prop, which means the ones you are running now are actually a 22" theoretical pitch. I think the best bet would be to get the solas amita 4 blades, although Dhadley may have a suggestion, if he does go with his suggestion as he has many years of experience as a prop tester for major prop manufacturers.


H

 

[rs2k]

Re: Repropping my cabin cruiser.

I have been wanting to change to an aluminum prop for some time. I don't like the idea of loosing an out drive if the prop hits something and I don't need the extra performance of an SS prop.

Do you know what kind of RPM I might expect at WOT with the following 14.25x19 four blade aluminum Solas Amita?

http://boatpropellers.iboats.com/OM.../6636/?**********=007421533&*******=961584571

 

[hwsiii]

Re: Repropping my cabin cruiser.

It is harder to say when you change to 4 blade props with a totally different blade geometry on twin engines, I believe you are going to get about 4,400 RPM, but that is subject to how much stern lift you are going to get out of the amitas. They could increase more than that because they might lift the stern a lot.


H

 

[rs2k]

Re: Repropping my cabin cruiser.

That sounds good. When you say that those blades increase stern lift will that mean that the boat would have a smoother transition to plane and a slower planing speed? That would be very beneficial for high speed trolling.

 

[hwsiii]

Re: Repropping my cabin cruiser.

If the solas amitas are zero degree rake or very low rake they should increase your stern lift, this is an art more than a science at the very end stage. If it was all science I wouldn't ask Dhadley for advice on the best prop. LOL
And yes it does mean that.

H

 

[rs2k]

Re: Repropping my cabin cruiser.

Very good! I look forward to hearing from Dhadley, but if I do not then I will go with your recommendation.

Do you know how this prop will affect WOT speed and cruising speed RPM? MY main use will be for trolling, but if I were to go on a 100 mile cruise would it be better to have another set of props for that purpose?

 

[hwsiii]

Re: Repropping my cabin cruiser.

Let's see what Dhadley has to say, hopefully he will see this.



H

 

[rs2k]

Re: Repropping my cabin cruiser.

I just ordered two Solas Amita 14.25x19 4 blade props.


The lake was really really choppy yesterday. About 1.5 to 2 foot wave heights. I was getting 3950 RPM with only two people and 50 gallons of gas at 43 MPH at full throttle.

The new spark plugs did make a difference. Both engines were singing together quite nicely at all RPM ranges and they had much better throttle response. I usually can't get them into harmony below 3600. Yesterday I had no problem syncing them at any RPM.

 

[hwsiii]

Re: Repropping my cabin cruiser.

That is very good to hear, it appears the engines are running very good now. It will be great when you get your props in and are able to see how they work for you.


H

 

[rs2k]

Re: Repropping my cabin cruiser.

I got the new props on today and have some very good results.

The boat performs much better with the Solas Amita 14.25x19x4 props. It now out performs my two little tri-hulls in almost every way. The engines now sing together at all RPM ranges. All the following speeds are GPS verified. The full throttle speed was taken during a full speed run into a 15 MPH wind and in light chop (about 1 foot wave heights.)

Idle speed:
The boat has much more bite at idles speed which makes low speed maneuvering much easier. Before the boat turned VERY slow when I put one engine in reverse and one in forward. Now it rotates at about twice the speed. It also turns very predictably when in reverse, but has a a bit more dog-walk to starboard then it did before. There is no noticeable dog-walk in forward with both engines engaged. With only the starboard engine engaged there is a very noticeable dog-walk to port. With only the port engine engage there is no noticeable dog-walk.

Displacement speed:
The boat responds quicker to helm inputs. The bow needed fewer corrections to keep from wondering back and forth. Neither set of props wondered when there was a wake behind the boat. The wake is noticeably smaller at displacement speeds. and during the transition to plane. I used to have two distinct rooster tails from the SS props on top of a big boat wake. The new four blades props just have one big (but smaller) wake.

Transition to plane:
It now transitions to plane MUCH more smoothly and predictably. Before there was a definite transition between about 8 MPH and 20 MPH. Any speed in between was very tricky to stay at. The boat either wanted to sink back into the hole or climb on top. Now I have no trouble keeping the speed to where I want it from 5 MPH (idle speed) to 13.5 MPH (2100 RPM). It's a little touchy between 13.5 and 18 MPH which around where the transition is, but I can keep it now on plane as low as 14 - 16 MPH with a bit of throttle management. To keep a speed between 5 - 13.5 MPH or a speed between 18 MPH and 45 MPH I nearly need to set and forget the engine RPM, although to keep the low planing speeds during a turn I do have to bump the throttle a bit. Before the set and forget speeds were 5 - 10 MPH and 20 - 42 MPH. Staying on plane at 20 MPH was also much more difficult than before.

Hole shot:
About half the time with the new props.

Single engine hole shot:
Still can't do it. Either engine stalls at 2500 RPM (Full throttle) on the nose while in the hole at about 10 MPH.

Cruising speed:
The mid range feels terrific. The throttle now responds nearly instantly and the speed stabilizes much faster than before. I was cruising at at 35 MPH at only 3200 RPM and the engines sing together quite nicely. 40 MPH sounded about the same while showing 3500 RPM. I was also amazed out how much more nimble the boat is. I really need to get a kill switch for it. I could very easily throw someone out of the boat. The boat just cuts through the water like a knife. This big old boat is more maneuverable than my 18' 305 powered tri-hull and my 17' 100 HP evinrude outboard tri-hull. It's quite fun to cruise along at 35 MPH and quickly cut over my own 2 foot wake. This boat just glides over it though. My tri-hulls pound on there own wakes.

Full throttle:
Here's where I have a little bit of a problem. The boat had no problems running at 45 MPH today and conditions were better than my last test with the SS props. The engines sounded like they were definitely going faster than with the SS props, but my RPM still only showed 3850 - 3900 RPM. I have tested the accuracy of the tachometers to 3200 RPM while on the trailer, but I have a feeling I have a two bad tachs. I'm not really sure. I will bring out my hand held inductive tach next time I go to the lake. Is it possible I'm only getting 3850 RPM and 45 MPH on the new props? The engines definitely sounded faster, louder, and smoother than with the SS props which is why I think the tachs may be lying.

Conclusion:
Results where much better than I expected even though I still couldn't get up on one engine. I am very happy with the new props. I need to confirm that my full throttle RPM is correct. If I am still doing less than 4000 RPM than I might want to move to a smaller pitch prop, but I think the 4 blade designs is a real winner for this boat.

 

[hwsiii]

Re: Repropping my cabin cruiser.

I am very glad to hear that, I do believe there is a problem with the tachs as you are turning the same RPM with these 19's as you were with the 21's, so please verify the RPM as the SS props had not only had a lot more blade geometry but the cup in the blades as well which theoretically adds another 1 in pitch to the props.
The dog walk is created by more torque and thrust that is produced by the increase in blade surface area from the 4 blades.
I don't quite understand the following statement, as you said you could plane at a lower speed now.

Staying on plane at 20 MPH was also much more difficult than before.

Get back with me after you verify the tachometers and lets see where we are, but it seems you are much happier with the 4 blades than the 3 blade SS you had prior to this. I still don't know whether you have a 1.41:1 or a 1.49:1 gear ratio, so I can't run some analysis that I would like to and reliably depend on the information from them.



H

 

[rs2k]

Re: Repropping my cabin cruiser.

I'm sorry, that statement was mistyped. I was trying to say that it's easier to stay on plane at 16 MPH with the 4 blades than it was to stay on plane at 20 MPH with the three blades. The boat is smoother in all bow attitudes and speeds. It used to take almost constant management to keep the boat planing at 20 MPH. Now it takes an RPM adjustment every few seconds to stay at 16 MPH and no adjustments at all to stay at 20 MPH. I can consider 20 MPH to be minimum cruise speed and 40 MPH to be max.

 

[hwsiii]

Re: Repropping my cabin cruiser.

That makes more sense. LOL

What doesn't make sense is there is no change in RPM, those SS props were a theoretical 22" pitch and we changed 3" in pitch with no change in RPM, and that makes no sense.


H

 

[rs2k]

Re: Repropping my cabin cruiser.

What about the extra 0.25 inch diameter, extra blade, and the fact that they're aluminum? Won't each one of those factors lower the RPM as well?

The engines do sound like they are running faster though. I will get back to you with the correct RPM ASAP.

 

[hwsiii]

Re: Repropping my cabin cruiser.

Yes it does, but theoretically speaking there is a 600 RPM difference between a 22" prop and a 19" prop. Usually there is about a 150 to 200 RPM difference between a 3 blade and a 4 blade prop. The 1/4" difference really is negligible and the difference in thickness of the blade between the SS and aluminum should not matter more than 100 RPM. The normal main differences between aluminum props and SS props is in the blade geometry which controls a boats attitude and handling characteristics as well as usually adds a little speed with the right blade characteristics. I have never seen a solas 4 blade so I don't know the blade geometry, that is why I couldn't say how much stern lift you would experience with that prop. If I am there to see the props and take a ride in the boat I am able to say what prop is the one that is just right for it, but doing it over the internet without ever seeing anything and not knowing the exact rake, progressive pitch, blade surface area and cup between the props makes it much more difficult. I have started building a database of the individual characteristics of each prop on the market, but it is very hard because the manufacturers don't tell you exactly what those characteristics are, they just give you public relations hype and that doesn't help me at all. LOL



H

 

[rs2k]

Re: Repropping my cabin cruiser.

The little bit I read about the Solas Amita is that it provides great stern lift and mid range performance. I can't really comment on the stern lift because I don't have a way to measure it, but the performance is "boat loads" better in the mid range... and the low end... and the top end. I can measure that performance with my tach and GPS.... and the seat of my pants.

I didn't really take much time to compare the props, but the Solas Amita looks like it has more rake than the OMC SS props I pulled off the boat. That surprised me a lot. In fact, I thought that I had the wrong props sent to me when I first pulled them out of the box.

When I look at this image and compare it to the Solas Amita:
http://i369.photobucket.com/albums/oo138/hwsiii3/DotsonPropellerRake.jpg

The Amita looked more like prop number 2 than prop number 1. In fact, the back of the blades are close enough to the back of the hub that both the back of the hub and the blade tips where put into the same packaging wall. If you look at the SS props in the image I posted earlier you can see the back of the blades are no where near the back fo the hub.

Here is an Amita 3 blade photo I found:
http://i74.photobucket.com/albums/i280/npd4432/3.jpg

 

[hwsiii]

Re: Repropping my cabin cruiser.

Higher rake implies more bow lift than stern lift, does it have any cup on the trailing edge of the prop, that creates stern lift, and they publicize it as a stern lifting prop. And the pictures I have seen of the 4 blade amita don't look to have that much rake.
Ok, I will be waiting on the test results for the tachs, so we know where we stand. Let me know please.


H