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bayonnebandit

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Posts: 114
Reply with quote  #1 
Model # E90DSLSRS
Serial # 05060702

Not sure if it should be in another section, but I have a general question and it pertains to my 90 so I will post it here. I have a 1976 Starcraft Aluminum 21ft boat that is very light for its length with a hull weight of approx 1100lbs. I run a viper 13 7/8 x 17 prop and I turn it at exactly 5200rpm according to my analog gauge set. With that in mind, I am able to get to about 36mph WOT according to a gps when I’m by myself with a full tank (19 gallons). Here is where I start to question things though. I have read on numerous accounts (other sites and on here) that boats (usually 17fters) turning that prop at that RPM should be getting about 41mph ... the rough speed you theoretically get when you do the math between prop pitch and rpm. So why am I slower? Is it hull length? Is it the hull design, being a deep V setup. I trailer it so there isn’t any growth and I have the engine up high so that the cavitation plate sits right at the bottom of hull/at the waterline at speed. I just can’t understand why I would be slower other than prop slip or my gauge set is showing the wrong RPM. To me, it would seem that if you can turn a certain pitch prop at a certain RPM you should roughly go the same speed no matter what. It would seem that weight, height of the motor, hull design would affect the size/type/pitch of prop you can turn but the overall speed of a prop at a certain RPM should remain constant. Am I not getting something here?

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21ft starcraft aluminum center console 2004 ETEC 90hp 

boscoe

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Reply with quote  #2 

If the propeller were a screw, with the threads screwed into a solid, then what you surmise would be correct. No matter what, for every turn of the screw it would move forward X distance. Within the limits of the materials of the screw and the substance that it is screwed into of course.

But a propeller is not a screw. Even though the propeller is sometimes called a screw. 

A propeller does not move itself (and the motor and the boat) by screwing its way forward as a screw would. A propeller takes a mass of water, imparts energy into it, and propels the water rearward at some velocity. Ever heard that for every action there is an equal and opposite reaction? The water being propelled rapidly aft results in a force (known as thrust) that moves the propeller, the motor and the boat forward. Or tries to anyway. If the boat is tied down the boat is not going anywhere, even at wide open throttle.

At wide open throttle the thrust force is as great as it can be. But the drag of the boat and the motor, increases as the length of the boat increases and as the weight of the boat increases. More weight/more length, more drag, less MPH. Less weight/less drag, more HP.

bayonnebandit

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Posts: 114
Reply with quote  #3 
Thank you for the insight, the boat being tied down was especially helpful. I still am at a little confused though. I am thinking of this example. Take two identically setup 90hp engines with identical props and put one on a 17ft boat and one on my boat. If my boat has more drag because of its length, weight, or any other factor, then that reverse energy would impart a higher opposite force on the prop causing more strain on the engine and therefore less RPM. The decreased RPM would result in less engine output force and therefore a slower boat. However, if in this example the resulting RPMs are identical, yet I still have a slower boat, then it would imply that there is additional slip happening at my prop. The thrust of the engine is being lost somewhere. If this is the case, can that lost energy be found? A second part to that question would be if slip has anything to do with engine output. Meaning, if there ever could be one, would a zero slip prop require more energy to turn than a prop with 10% slip. Or, in other words, if I eliminated that slip, would my RPMs diminish because the prop has more gripping force in the water.

My head hurts

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21ft starcraft aluminum center console 2004 ETEC 90hp 
PhilH12

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Reply with quote  #4 
How are you Trimming?  The general rule is to trim up while watching both RPM's and boat speed.  When the rpm's increase with no increase in speed then drop back down just a bit.  That's your Sweet Spot for that motor height and water conditions.

Speaking of which, where is your AV plate when you're up on plane?  It wants to be above solid water and getting splashed.  If it's too low it's like dragging a sea anchor behind your boat (or being tied to the dock).

EDIT:  If your engine is too low and you raise it up appropriately, you may have to go up in pitch to come back to the 5,200 rpm's and Do hang on at WOT.  [wink]  Also, if you're too high, you can be spinning the prop and pushing against aerated water which will also slow you down by not enough of that Thrust mentioned by Boscoe.

Phil

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20' Weeres fishing pontoon
90 HP E-Tec E90DPLSUM  05184332
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boscoe

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Posts: 564
Reply with quote  #5 
Quote:
Originally Posted by bayonnebandit
Thank you for the insight, the boat being tied down was especially helpful. I still am at a little confused though. I am thinking of this example. Take two identically setup 90hp engines with identical props and put one on a 17ft boat and one on my boat. If my boat has more drag because of its length, weight, or any other factor, then that reverse energy would impart a higher opposite force on the prop causing more strain on the engine and therefore less RPM. The decreased RPM would result in less engine output force and therefore a slower boat. However, if in this example the resulting RPMs are identical, yet I still have a slower boat, then it would imply that there is additional slip happening at my prop. The thrust of the engine is being lost somewhere. If this is the case, can that lost energy be found? A second part to that question would be if slip has anything to do with engine output. Meaning, if there ever could be one, would a zero slip prop require more energy to turn than a prop with 10% slip. Or, in other words, if I eliminated that slip, would my RPMs diminish because the prop has more gripping force in the water.

My head hurts 

There is a lot going on with propellers. Enough to make ones head hurt.

Slip. I hate that term. But I understand why it is used. In my view slip is not going to have anything to do with the output of a motor. The motor is going to make X power. Power which is based on the manifold pressure and the RPM of the motor under a load. That power is transmitted to the propeller. The propeller converts the power into thrust. The thrust propels the boat. Ideally 100% of the power would be converted to thrust but usually the output of the propeller is less due to a loss of efficiency. A loss for a number of reasons.

To be continued.  
Tee2Sea

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Reply with quote  #6 
Engine RPM is what we typically measure, but what counts is propeller RPM.  Are you sure that you have the same gear ratios as those you are comparing?  Evinrude marketing material is not accurate. You need to check the service manuals and be sure which lower unit that you have.  On my 115HO there are several combinations, from 2.00:1 to 2.36:1.  Whats it all mean, about 800 RPM difference at cruising speed, so one set up is doing 4200 rpm and another needs 5000 rpm to achieve the same speed.
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115 HO  mfg 2018
A115SHLAFL    05530167
2003 Hydra Sports 180CC  TP13.25 17 Mach4
Huey

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Reply with quote  #7 
A lot of variables and can be a waste of time comparing different hulls, even the same model hulls, open may have a hook in the bottom that does not perform as well as the exact same model hull as the next one.

A few things, as pointed out your age engine has a different ratio from I-3 90HP's from the last 7 odd years. The new ratio really woke up the 90HP I-3. Also to me, that looks too low, you need to make sure you can clearly see the AV plate when trimmed correctly. need to then confirm via Ev diag or digital tacho your true WOT RPM, as mentioned many time analogue tachos can be out. I like the Rogue prop on stern heavy boats like you console model.
Huey

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Reply with quote  #8 
A lot of variables and can be a waste of time comparing different hulls, even the same model hulls, open may have a hook in the bottom that does not perform as well as the exact same model hull as the next one.

A few things, as pointed out your age engine has a different ratio from I-3 90HP's from the last 7 odd years. The new ratio really woke up the 90HP I-3. Also to me, that looks too low, you need to make sure you can clearly see the AV plate when trimmed correctly. need to then confirm via Ev diag or digital tacho your true WOT RPM, as mentioned many time analog tachos can be out. I like the Rogue prop on stern heavy boats like your console model.
boscoe

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Reply with quote  #9 
Quote:
Originally Posted by bayonnebandit
Thank you for the insight, the boat being tied down was especially helpful. I still am at a little confused though. I am thinking of this example. Take two identically setup 90hp engines with identical props and put one on a 17ft boat and one on my boat. If my boat has more drag because of its length, weight, or any other factor, then that reverse energy would impart a higher opposite force on the prop causing more strain on the engine and therefore less RPM. The decreased RPM would result in less engine output force and therefore a slower boat. However, if in this example the resulting RPMs are identical, yet I still have a slower boat, then it would imply that there is additional slip happening at my prop. The thrust of the engine is being lost somewhere. If this is the case, can that lost energy be found? A second part to that question would be if slip has anything to do with engine output. Meaning, if there ever could be one, would a zero slip prop require more energy to turn than a prop with 10% slip. Or, in other words, if I eliminated that slip, would my RPMs diminish because the prop has more gripping force in the water.

My head hurts


If all is equal (same boat, same boat weight, same boat CG, same motor, same motor trim angle, same propeller, same atmospheric conditions, etc.) but one boat is faster than the other then I would surmise a loss of propeller efficiency. Due to propeller differences. Although both may be of the same brand, model and pitch.
boscoe

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Posts: 564
Reply with quote  #10 
Quote:
Originally Posted by bayonnebandit
Thank you for the insight, the boat being tied down was especially helpful. I still am at a little confused though. I am thinking of this example. Take two identically setup 90hp engines with identical props and put one on a 17ft boat and one on my boat. If my boat has more drag because of its length, weight, or any other factor, then that reverse energy would impart a higher opposite force on the prop causing more strain on the engine and therefore less RPM. The decreased RPM would result in less engine output force and therefore a slower boat. However, if in this example the resulting RPMs are identical, yet I still have a slower boat, then it would imply that there is additional slip happening at my prop. The thrust of the engine is being lost somewhere. If this is the case, can that lost energy be found? A second part to that question would be if slip has anything to do with engine output. Meaning, if there ever could be one, would a zero slip prop require more energy to turn than a prop with 10% slip. Or, in other words, if I eliminated that slip, would my RPMs diminish because the prop has more gripping force in the water.

My head hurts

Possibly the motor will produce less power given that the additional drag of the larger boat will result in the WOT RPM being less. But it could result in the motor making more power. Depends upon the starting point of the WOT RPM.


steelhead

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Reply with quote  #11 
Need more mules on the back.  The I3s are not known for high hole shot torque on larger boats.
Loaded down with 4 on board, ice chests and refreshments, doubt it will pull 30

Most of those old Starcraft 21s were armed with Merc6 115 and 150 Tower of Powers or V4 'Rudes 140hp class and V6 150>200s.  Carb'd motors all, so they were way over labeled HP at WOT
Then they fly and scream

For a rivet boat, they are tough even in rough ocean wind chop.
Fill the bottom under the plywood with medium density foam and they will take more than the skipper can.  Knee and back busters but will bring you in.

Is this hull a converted 140 Chevy 4 cylinder inboard?  Were popular but slow.
The transom is 2" thick plywood, heavy stern.

Have somone take a side picture with the boat running WOT plane trimmed out on smooth water.  We'll take a look at the ride angle
bayonnebandit

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Posts: 114
Reply with quote  #12 
I do know the ins and outs of properly trimming the boat in order to get it to the proper RPM given the conditions at the time. The boat is pretty unique in that it takes quite a bit of trimming up to ventilate the prop at WOT. Also, I never experience anything like porpoising or loss of grip when coming around a turn.

I will try to get out on the water and take a picture of the cavitation plate but I have it on the highest setting it can go before I have to take the motor off, plug the current holes, and drill new ones (read as : pain in the rear, literally). The plate sits level with the keel I know that because I moved the engine up a few years ago. That being said, it’s a rather sharp v so it’s possible the plate is slightly submerged at WOT and trimmed out... I’ve never actually got back there and looked while running.

The boat is a 1976 Starcraft Mariner CC I completely renovated a number of years ago. It was my fathers boat and he passed away when I was young. It sat in our yard untouched for about 20 years before I decided I give her the face lift she deserves. I believe it to be slightly heavier than the original build because I used trex decking as opposed to plywood for the floor so that it would last forever. It’s always been an outboard motor on it with 1 1/2” plywood transom (actually double layer 3/4” marine grade ... I know because I replaced it). I do take it out in the ocean off of NJ and she does well even into 5ft seas (a little scary) although it is very hard on the back/knees as you have said Steelhead. She also will pop rivets from time to time but I have a sealed blind riveter to take care of that when it happens. Interestingly enough though, my hole shot is extremely impressive. I can rip the arms off of a skier before they are able to get up. Also, weight doesn’t really seem to affect my speed greatly which is another strange anomaly. Five adult men loaded with gear and enough beers to kill a horse and I can get 34. Yet by myself no cooler and I hit 36. MAKES NO SENSE.

I’ll see if I can get some help and take a picture/video of the boat from shore while I’m underway. I appreciate all the feedback.

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21ft starcraft aluminum center console 2004 ETEC 90hp 
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