8000 lb cruiser twin merc 260's

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

oh well kinda figured as much... I think I've pretty much figured this out anyhow...... I looks like there is a minimum clearance needed between the prop tips and the anti cav plate.... it also looks like this distance goes up with rpm.... has to do with induced drag on the prop.... so a lower ratio leg can run a larger diameter but the pitch has to go up to the point that it is less efficient than the smaller diameter prop with less pitch....

Thanks anyhow

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

I found something!..... I was looking in my mercruiser factory service manuals yesterday and came across a prop selection table.... These books are pre alpha but were suggesting up to 16" diameters on large low powered boats with r/mr drives...... so there is some possible truth to my theories.... this data did not cover 520 hp twin drive boats but at least it is a tad more data to work with and since suggested prop diameter went up with hull weight and not just hp I feel that this is still worth looking into

I know the 16"x14" prop worked very well on my carver but I was concerned about efficiency since I was not able to do adequate comparison testing

Now this is an old manual and I'm certain that prop design has advanced since it was printed but it looks like the most directly connected variable is water speed..... the lower the top speed the larger the diameter of the prop..... now at this point this is all assuming a 1.5 ratio.... I'll save that variable untill I have this one figures out

 

[hwsiii]

Re: 8000 lb cruiser twin merc 260's

Kevin, you might try reading this, as I believe it will answer a lot of the questions you have.

http://forums.iboats.com/showthread.php?t=381820



H

 

[dazk14]

Re: 8000 lb cruiser twin merc 260's

Smokeonthewater,

Did you decide on props and do any testing? Had similar questions about going the larger diameter route.

Fyi - My Volvo 290 drive allows for a 16' max diameter prop(per Volvo) and that leaves only 3/8" (I measured) space between blade and cavitation plate at 16"dia..

Most 290 Volvo boats are running both 14" and 15" diameter blades for most pitch sizes. I had a similar question to yours about jumping to a higher diameter smaller pitch or vice versa, but I'll ask that in another thread.

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

I haven't managed to get mine on the water yet but it looks like there is a real connection between large boats and large props....

 

[smokeonthewater]

Re: 8000 lb cruiser twin merc 260's

hey folks.... I figured I'd update this with the little bit of info I have...... at present I'm getting 45 mph with the michigan wheel 14.5x17 props... these are old pressed hub props..... since I started looking into this I did manage to get the boat on the wayteer and then became POOR.... I won't be trying any different props this season but I just thought I's add this info for anyone who cares..... BTW the fuel mileage is as expected .... I calculated it to be exactly "horrible" LOL

I'm learning to enjoy idle speed a LOT


H said earlier that he thought I'd benefit from a certain 4 blade prop for more stern lift because this boat would be stern heavy with the twin 260's.... I have no doubt that will turn out to be correct.... As it is I really don't have a slow cruise speed.... about 2600 rpm and 25 mph is my slowest useful planing speed right now.... Also the boat WILL plane out fairly quickly but with 4 or 5 people on board (of course they all want to sit in the back) it's a full throttle event.... I'll probably pick up those 4 blade props next season (If I'm not still poor lol) and try them out.