Houseboat Prop

[river rat 26]

Howdy you all. I put a new 90 evinrude on my boat (26' tritoon with a very heavy cabin built on top) I started out with a 16 stainless but could not get the rpm above 4000 so I got a 12 stainless and went up to 4500, I liked this prop it had lots of cup and pulled real good but I was still on the bottom of the recomended rpm range for the e-tec and no one around had any thing lower in pitch. I called BRP and the tech recomended a 9 which I ordered and have on there now. Got the rpm up to about 5200 which is good but this big ugly elephant ear looking aluminum prop with no cup has none of the things I liked about the 12 I had on there. That prop just pulled so much better so I called and talked to my prop builder and he is thinking of going to a 7 or 8 in stainless with the cup I loved on the other prop which sounds great but I need to know what you all think befor I spend this non refundable cash on this prop. He says when it comes in it will be mine with no returns cause it is custom. What will the change be going from aluminum to SS and no cup to cup, I can go as high as 5500 rpm so says the BRP tech. Thanks in advance.

 

[Silvertip]

Re: Houseboat Prop

Without any speed data it's tough to make a prediction and seat of the pants feeling can be misleading. Actually for a heavy boat like yours, the "elephant ear" four blade prop should be better suited to the job as it provides better push both forward and backward when needed due to the extra blade, their shape, and extra blade area. Cup vs no cup is just one of many variables in prop design. Although I've not done a lot of prop testing on pontoons, my experience with planing hulls is that a given cupped prop (15P for example, turns about the same rpm (give or take a little) as an uncupped 17P. Combinations of cupping, blade shape, and rake determines the degree of bow or stern lift which really aren't much of a concern on your 'toon. I guess my feeling is the proper four blade (cupped or no cup) is better in your application than a three blade with cup. What that prop would turn out to be in your exact application is a tough call.

 

[steelespike]

Re: Houseboat Prop

Guess I'm a little late.
I'm a little outside the sofisticated prop technology but before going for that prop you may want to try a 4 blade or wait for an opinion here on a 4 blade.
It would help some if you could supply speed onfo.Does the tri toon do anything close to planing?If not that 4 blade may help.Also you may want to work on the setup.With all that weight on there using the 12" prop you may be able to raise the motor a couple of holes if your not getting any ventilation the motor is too low, of course careful trim may help.Might get you a couple of hundred rpm.Then you can make a better decesion on the prop.You might consider taking some of the pitch and maybe a little cup out of the 12.Perhaps a little cup added to the aluminum.Heck of a lot cheaper than a custom built prop.

 

[river rat 26]

Re: Houseboat Prop

I had not thought about a four blade prop, I guess this may be a good application for that...I have tried moving the mtr up and down with no noticable differance, and triming up with any of the props I have tried makes no more than about 50 rpm differance. the boat used to run about 16mph with the old 75 model 85hp on there, with the new mtr it is running 23mph and yes it does plane (as much as a toon can). The 23mph is concistant with all the props I've tried of couse the lower pitch gets me there faster...So what you all are telling me is a drop of two degrees of pitch to get the handling advantage of a prop with some cup sounds right to you? All so is there a standard rpm shift changing from aluminum to SS?...I have had no luck in finding any props over the counter in a lower pitch than 9 this is why I am assuming that I have to have one built. Oh on the 12 it was a borrowed prop from local prop guy, if I want him to change it I have to buy it. Thanks again.

 

[Silvertip]

Re: Houseboat Prop

The process of raising an engine on the transom allows for less drag and to get the most out of high performance props. But on a pontoon/houseboat that is not a major concern because you are not going fast enough for drag to have much of an impact. There are two schools of thought on what to do when switching from SS to AL or vice versa. One school says keep pitch the same others say change pitch by one step. The problem is that an AL prop may not be the same exact design as the SS so the performance change can be hard to predict. I just ran the numbers on a 9P prop through a prop calculator and at 13% slip and 5500 RPM it showed about 21 MPH so I think 9P is close to what you need. The next thing to check is to make sure the throttle is opening all the way and that the engine is actually making full power. It would not be unusual for either or both situations to be present. I didn't do an extensive search but I did turn up a Solas Amita 13P four blade aluminum prop (2413-133-13). Given that you got 4500 RPM with a 12P SS with lots of cup, I would suggest you search for any aluminum four blade in 11P range and not worry about cupping. The Solas I mentioned is $100 and it appears to have some cup and would be a relatively inexpensive way to start fine-tuning you setup. In the end it could serve as a spare prop should the need arise. I would hate to see you spend a bundle on a special stainless prop and still not have it work for you. Besides, it would be tough to sell because it is special. Wish I could help more but we are sort of shooting from the hip here. I am reasonably certain however that you indeed need a pontoon-four blade. Here's a 14x9 three blade AL. http://www.usboatsupplies.com/mm5/m...de=USBS&Product_Code=763300&Category_Code=BAL and here's a Johnson/Evinrude pontoon AL three blade: http://www.usboatsupplies.com/mm5/m...de=USBS&Product_Code=177201&Category_Code=BAL

 

[river rat 26]

Re: Houseboat Prop

thanks for the links Silvertip, They look much like what I got from the dealer (the 14x9 that I am running now) except for the ring at the end of the barrel, what does that do? Am I right in assuming that the extra blade on a four blade prop will indeed increase pulling power? The reason I am stuck on cup is that I liked the SS 12 I tried so much (other than the RPM) It is hard to explain but it just felt better than this 14x9 AL that I have on there now. It held the bow up at lower speeds, really it seemed to raise the bow just off idle (this is important on my big heavy boat) also it backed off the trailer with very little throttle. I suppose this could have been blade design as much as cup. What I had in mind was getting my prop guy to duplicate that prop in a lower pitch but he wants me to tell him what I want which is understandable considering the cost involved, hence my questions about AL to SS and cup no cup etc. I like the four blade idea but I'm hesitant about adding another variable at this point. If I lost top speed it would't bother me at all I'm just looking for that lift and of course I need the rpm as close to 5500 as I can get so this beautiful new mtr. will last as long as my old reliable 85 did.

 

[Silvertip]

Re: Houseboat Prop

Let's regroup. Unless you consider ALL of the factors in a prop design, applying just one or two characteristics between what amounts to two totally different designs becomes nearly impossible to predict what the outcome would be. But to answer your question, yes (usually) a four blade is better than a three blade out of the hole but gives up a little on the top end. But again, that is no guarantee because we are taking about a pontoon that, while it may sort of get on plane, it is not the same as a "V" hull so the four blade may outperform the three blade in all areas. With the four blade you may be able to trim up a little more to raise the bow during hole shot. You seem convinced that the three blade you tested is what you are happiest with. In that case, you should continue to tweak that design and not try to mix design characteristics of other props.

That plastic ring on Johnson/Evinrude props is called a diffuser ring. You will note most props have some sort of a flare at the end of the hub. As an aside, I took delivery of a new pontoon and I'm betting the 11P, cupped three blade aluminum on my Suzy 4-stroke will be replaced with a four blade before the summer is over and this boat weighs nowhere near what yours does.

 

[river rat 26]

Re: Houseboat Prop

Thanks again Silvertip, yes I should stay with what I know and work from there...I was reading some old e-tec threads and it seems these motors don't like 4 blade props anyway. Thinking on having the prop builder (who I trust completely) order a three blade 14 x7 cupped like the one I liked with a simular blade design. All the reading I've done on here and other websites is telling me that an aggressive cup is like giving up 1" of pitch and I could use another 300 RPM so a 7 sounds right. All my testing has been with a very light load, just me and the dog. I think this will get me where I want to be...I'm surprised to find that I can't find any props pitched lower than 9 anywhere. I know my boat is a little unusual but it seems with others pushing barges and such there would be someone making them. Happy Boating !

 

[Silvertip]

Re: Houseboat Prop

That's why I suggested making sure the throttle is opening all the way and that the engine is actually making full power. It just doesn't seem like you should need a 7P prop as the prop calculator puts revs at nearly 6100 with 13% slip at 20 MPH.