Prop Size Dilemma?

[TonyC]

All,

Pulled the props on our 1998 350 Mariner. Prop shop clean and scanned and told me that I could get another year or so
out of them after a tune-up. However, due to age and that they are brass, should not expect much more life out of them.

They were concerned about the props and boat configuration. Current props are 4 blade, 20 x 23. Boat has twin 350 MAGs MPI at 300 HP. with V-Drive 1.5:1 ratio. They tell me that the "number" crunching suggests that we should use 4 blade, 20 x 21. They called and did confirm with Carver that the boat came with the props that I took off. The prop shop asked what my WOT RPM is and, while I did open it up a couple of times it was not for a long duration. I believe that the tachs were over 4000 but by how much I do not recall.

We rarely cruise more than at plane ... around 3000 - 3200k. What can be a problem and is a concern is at low RPM while docking. We need to fight a current and wind changes. With the current props the boat responds sluggishly at best without adjusting the throttles (which I prefer not to do while docking). The way I understand prop and prop design, the 20 x 21 should provide more thrust at lower RPM's providing better response while docking.

Any thoughts on this? Should I pull the trigger on the suggested new props (brass and other alloy combination at $3100 for both) or go in a different direction. We love this boat and plan to keep a long time. Thanks!

TonyC

[bud37]

So here are the numbers from our boat,just like yours but 2001.... michigan dyna quad 20 X 22 4 blade....Engines are 6.2 ( A little more power)...just a little info, don't know if this helps or hinders......I would be interested to know how the props wear out.....

[TonyC]

Qr Bbpost So here are the numbers from our boat,just like yours but 2001.... michigan dyna quad 20 X 22 4 blade....Engines are 6.2 ( A little more power)...just a little info, don't know if this helps or hinders......I would be interested to know how the props wear out.....

Thanks bud37 for this information. Same boat with more power but props with lower pitch. Assume your low end control such as that needed for docking is OK? Sounds like my prop shop may be on to something!

TonyC

[waybomb]

How does a prop wear out?
I am curious as well.

[mjk1040]

Well carver had issues with power plants some years and would change the props to keep RPM's at WOT within engine specification. Props could become worn out do to electrolysis or erosion. Since you have a Mariner and the design of the haul makes it skate easily in the wind and currant, proper propulsion docking at low rpms is an issue of great concern when choosing props along with performance at WOT and cruising speeds. WOT should max out around 4,400 rpms. That is why carver most likely put the props on that you took off. Maybe take a look at other Mariner props on the dry? Personally I'd stick with what carver put on the boat when it was new. Mike

[bud37]

Qr Bbpost

Qr Bbpost So here are the numbers from our boat,just like yours but 2001.... michigan dyna quad 20 X 22 4 blade....Engines are 6.2 ( A little more power)...just a little info, don't know if this helps or hinders......I would be interested to know how the props wear out.....

Thanks bud37 for this information. Same boat with more power but props with lower pitch. Assume your low end control such as that needed for docking is OK? Sounds like my prop shop may be on to something!

TonyC

Docks ok, but I will add something here in regards to the handling.....previous boat we had was about the same size with a flybridge but had a keel and the props hung lower, now that boat bit a lot better at idle ( way smaller props) than does the Mariner, like Mike said and I find, the bow slides around easily.....dont think the props will help much. Maybe this is why you see thrusters on some of these...just my opinion.....

[TonyC]

Qr Bbpost Well carver had issues with power plants some years and would change the props to keep RPM's at WOT within engine specification. Props could become worn out do to electrolysis or erosion. Since you have a Mariner and the design of the haul makes it skate easily in the wind and currant, proper propulsion docking at low rpms is an issue of great concern when choosing props along with performance at WOT and cruising speeds. WOT should max out around 4,400 rpms. That is why carver most likely put the props on that you took off. Maybe take a look at other Mariner props on the dry? Personally I'd stick with what carver put on the boat when it was new. Mike

Thanks Mike for your input.

A couple of other posts inquired as to how props wear out. I improperly use the term "wear out" instead of "degrade". Your post hit on two of the most common reasons ... electrolysis and erosion. The other, from what my prop shop share with me, is that props, especially brass, flex at higher RPM's. After 20 years this flexing caused fatigue particularly near the shaft.

The jury is still our as to what I will do ... go with 20x23 as original or 20x21 as recommended by the prop shop. I do know what to expect with the original 20x23's. Not sure what improvements in all areas the 20x21's would provide. Also, not sure if I want to go with the less expensive brass or more expensive mix alloy. Not sure what the difference in price will be. I do know the the original brass lasted about 20 years. Think I need to ask prop shop what the different combinations and related costs will be prior to making a decision.

Thanks again for your input.

[TonyC]

Qr Bbpost

Qr Bbpost

Qr Bbpost So here are the numbers from our boat,just like yours but 2001.... michigan dyna quad 20 X 22 4 blade....Engines are 6.2 ( A little more power)...just a little info, don't know if this helps or hinders......I would be interested to know how the props wear out.....

Thanks bud37 for this information. Same boat with more power but props with lower pitch. Assume your low end control such as that needed for docking is OK? Sounds like my prop shop may be on to something!

TonyC

Docks ok, but I will add something here in regards to the handling.....previous boat we had was about the same size with a flybridge but had a keel and the props hung lower, now that boat bit a lot better at idle ( way smaller props) than does the Mariner, like Mike said and I find, the bow slides around easily.....dont think the props will help much. Maybe this is why you see thrusters on some of these...just my opinion.....

Thanks bud37! You are right on regarding the bow being squirrely. Possibly why Carver designed the Mariner to be a tad bow heavy?

[mjk1040]

We had and loved a '86 Mariner, but the haul design of this model boat made it squirrely at low rpm maneuvers especially in the wind or current. Had our sights set on upgrading to a newer Mariner 10>12 years or so, until I had a chat with a fellow boater and neighbor at the marina who had two Mariners basically the same year spread. So I asked him why he changed from the Mariner to the 356 AC/MY, he said Mike you can steer this boat! We then this 355 AC/MY and what a maneuvering difference. My wife can steer the boat in and out of the marina even after I spent days teaching her how to do it on the Mariner with just the engines. Like night and day! Mike

[waybomb]

I have a spare set of 22 x 22 x 1 3/8 you could try if you want.
I'm almost next door to Pier33.