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Propping a Honda BF75 on a Montauk
Paul Graham	#1  Posted on 09/04/09 - 2:53 PM
Member Personal Page Posts: 123 Comments: 0 Joined: 05/18/07	I need some advice, I had a 1970 16' hull rigged Montauk-style with a 75 HP Honda, turned 5200 RPM, now I took everything off that hull and put it on a 1978 Montauk with a 4" fixed (Bob's Machine Shop) jack plate, motor is on top hole and not slipping. I'm only getting 4700 RPM and I have to trim way up to get that. Any suggestions or thoughts on what's going on here? Thanks, PG Edited by Tom W Clark on 09/06/09 - 10:45 AM
Joe Kriz	#2  Posted on 09/04/09 - 2:58 PM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	Don't understand, "Motor is on top hole" ??? Is the motor mounted all the way up and the bolts are in the bottom holes of the engine bracket? Or, Is the motor mounted all the way down and the bolts are in the top holes of the engine bracket? After we know that, then we need to know more about the jack plate or where the cavitation plate is in reference to the bottom of the hull. Why a jack plate? So you didn't have to drill anymore holes in the transom? http://www.whalercentral.com/articles...icle_id=82
kamie	#3  Posted on 09/04/09 - 6:05 PM
Member Personal Page Project Albums Posts: 2975 Comments: 3 Joined: 11/04/05	Paul, The outboard is in the top hole on the jack plate, but which hole on the outboard mounting bracket, top as well? I believe your engine is too low, you need to move up to the second, possibly the third hole on the engine bracket. If you look at your outboard when running at speed, I believe you'll find that the cavitation plate is buried and not riding above the water.
Paul Graham	#4  Posted on 09/04/09 - 8:34 PM
Member Personal Page Posts: 123 Comments: 0 Joined: 05/18/07	The motor is all the way up, bottom hole on the motor. Prop is a stilletto 13 3/8 x 19, I've had the motor down 2 holes and get the same RPM's. I mainly put the bracket on to get the motor off the refinished hull, it drools a little rust from around the bracket at the top of the vertical pivot tube
Bo Neato	#5  Posted on 09/05/09 - 4:35 AM
Member Posts: 79 Comments: 0 Joined: 09/08/07	First, there's the hull difference. Aren't the 16s a little slicker in the water than the smirked hulls? The 16 also may have weighed a little less if the console and seat you used was Todd or another aftermarket. Any bottom paint on either? Maybe the 17 is a little more damp? 500rpms is a lot to lose, but many factors will add up and exacerbate the issue aside from hanging the motor. The prop seems to be maybe a bit steep as well. I know Tom W Clark has recommended Stilletto props that are 13 1/4 x 15 for Montauks with 90 hp OMC/BMC motors. I'd ask him, he's a prop guru and his advice was spot on for me. Don't get discouraged, you'll get it. Edited by Bo Neato on 09/05/09 - 10:32 AM
Paul Graham	#6  Posted on 09/05/09 - 6:12 AM
Member Personal Page Posts: 123 Comments: 0 Joined: 05/18/07	Thanks Bo, I would love to here what Tom would say about my situation. Tom, would you please give me some advise? I borrowed a 13 x 17 prop from a friend last night and will try that today, I also will take a closer look at the tach, I have a couple of spare tachs I can compare, thanks again
kamie	#7  Posted on 09/05/09 - 6:13 AM
Member Personal Page Project Albums Posts: 2975 Comments: 3 Joined: 11/04/05	Paul, where is the cavitation plate in relation to the keel?
Paul Graham	#8  Posted on 09/05/09 - 6:39 AM
Member Personal Page Posts: 123 Comments: 0 Joined: 05/18/07	It is 3" above the keel, but it does not appear to slip or blow out when I nail it.
kamie	#9  Posted on 09/05/09 - 7:26 AM
Member Personal Page Project Albums Posts: 2975 Comments: 3 Joined: 11/04/05	If the plate is out of the water on plane you may just have to drop back to a 17pitch prop.
Tom W Clark	#10  Posted on 09/05/09 - 7:49 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	You are overpropped. Use a lower pitch propeller. Kamie wrote: If the plate is out of the water on plane you may just have to drop back to a 17 pitch prop. No, that is illogical. He needs to drop pitch because the engine speed is too low at WOT. It has [almost] nothing to do with where the AV plate is. Having the (AV) plate out of the water is a good thing and increases, not decreases, the RPM.
kamie	#11  Posted on 09/05/09 - 8:14 AM
Member Personal Page Project Albums Posts: 2975 Comments: 3 Joined: 11/04/05	Tom W Clark wrote: You are overpropped. Use a lower pitch propeller. Kamie wrote: If the plate is out of the water on plane you may just have to drop back to a 17 pitch prop. No, that is illogical. He needs to drop pitch because the engine speed is too low at WOT. It has [almost] nothing to do with where the AV plate is. Having the (AV) plate out of the water is a good thing and increases, not decreases, the RPM. Tom, I am aware that having the plate out has nothing to do with the prop pitch, it is a confirmation that the outboard is mounted at the right height. With a jackplate and a 4 inch setback 3 inches above the keel on the trailer may or may not put the outboard at the correct height on plane. logically one of two things are wrong, either the outboard is too low or the pitch too great. Confirming that the plate is out of the water on plane, points to the pitch of the prop and if that is the case, I would suggest he drop to a 17pitch. If the plate is not out of the water at speed, then I would raise the outboard if possible, if it is not, then his only option is to drop down to a 17 pitch prop.
Paul Graham	#12  Posted on 09/05/09 - 9:41 AM
Member Personal Page Posts: 123 Comments: 0 Joined: 05/18/07	ok, I borrowed a 13x17 prop, I'm goinng to try it today. What will a 3/8" drop in diameter do? Thanks
kamie	#13  Posted on 09/05/09 - 12:41 PM
Member Personal Page Project Albums Posts: 2975 Comments: 3 Joined: 11/04/05	Paul, See if the pitch gets you closer to max RPM's at WOT. Once you nail down the pitch, then you can look at different props. I believe that your Honda should run between 5000-6000 at WOT with max HP at 5500. Depending on if the 17 puts you in the ball park, you might drop down to a 15 and get closer to 5500 to 6000RPMs.
Paul Graham	#14  Posted on 09/05/09 - 1:42 PM
Member Personal Page Posts: 123 Comments: 0 Joined: 05/18/07	I have no way of determining my speed, what will a 2' drop in pitch do to my top end?
Joe Kriz	#15  Posted on 09/05/09 - 2:17 PM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	Paul Graham wrote: ok, I borrowed a 13x17 prop, I'm goinng to try it today. What will a 3/8" drop in diameter do? Thanks Paul, We have really no choice in selecting a diameter in a prop basically. We can only choose the pitch... 15 pitch, 17 pitch, 19 pitch, etc.... The engineers figure out the diameter according to the pitch for a particular manufacturer. Different prop manufacturers even have a different diameter than other manufacturers even if they are all 17 pitch. We can't say we want a 13 1/8 x 17 or a 13 1/4 x 17 or a 13 1/2 x17 The general public buys props by the pitch and brand... The RPM's is what you really want to look at rather than speed. Your engine manual states that the engine should max out between certain RPM's at top speed... Whatever the top end speed is, that is what you get... However, generally the smaller pitch will give you a better hole shot. The larger pitch will give you more top end speed. Being over propped isn't good as your engine will never get up to the recommended RPM's when running and isn't good on the engine as it is working too hard. Edited by Joe Kriz on 09/05/09 - 2:21 PM
Paul Graham	#16  Posted on 09/06/09 - 9:06 AM
Member Personal Page Posts: 123 Comments: 0 Joined: 05/18/07	Launched the boat this morning, I can get a solid 5100 rpm and occasionally hit 5200, I'm sure I can have my 19" prop changed to 17", should I shoot for 16" ? Thanks
Tom W Clark	#17  Posted on 09/06/09 - 10:41 AM
Member Personal Page Posts: 4280 Comments: 7 Joined: 09/30/05	Paul, Can you tell us exactly WHAT 13" x 17" propeller you tried? You say you have a 13-3/8" x 19" Stiletto, but Stiletto does not make propeller in that size. Can you confirm the part number? At nay rate, I almost never recommend changing pitch of an existing prop, an NEVER as much as 2 inches of pitch. It is always much better to just get the prop that you really need.
Paul Graham	#18  Posted on 09/06/09 - 4:51 PM
Member Personal Page Posts: 123 Comments: 0 Joined: 05/18/07	Tom, I pulled the prop off a 130 Yamaha, it's a 13x17-K2, #805. Mike Hood's prop shop in Houston did not seem to think it was a big deal changing pitch as much as 2 inches. He actually recommended dropping 4" in pitch, but said he could not change a prop that much. Don't you think a 15" prop would to much of a drop, especially after seeing the improvement of a 2" drop in pitch. Why are you against changing the pitch on a prop? Thanks
Joe Kriz	#19  Posted on 09/06/09 - 5:16 PM
Site Owner Personal Page Personal Album Photo Albums Project Albums Posts: 11447 Comments: 452 Joined: 03/18/05	Paul, What does the prop off of a 130 Yamaha have to do with this discussion about the prop for your Honda????? Or am I missing something??? We need to know the numbers on the prop you have now on the Honda 75 this discussion is about... I have 3 props for my boat.... 15 pitch, 17 pitch, and 19 pitch.... These are all OMC props for my OMC 150hp motor... The 15 pitch gives the best hole shot and the 19 pitch gives the best top end speed. I decided to keep the 17 pitch prop on all the time as it is a happy medium for my needs. However, there is a big difference between brands of prop and their pitch or aggressiveness.... You need to find the right pitch prop to bring your RPM's to within the guidelines of your motor manufacturer. I personally wouldn't change pitch on a prop at all. Rather buy a different prop and keep the other as a spare or sell it or trade it in...
kamie	#20  Posted on 09/06/09 - 5:43 PM
Member Personal Page Project Albums Posts: 2975 Comments: 3 Joined: 11/04/05	Paul, Based on your test results with the 17pitch your prop guy is correct, you should drop to a 15 pitch. The range for that outboard is 5000-6000 at WOT with the outboard making 75HP at 5500RPM's. That should be your target RPM's. As for changing the pitch, since you should really drop 4 inchs, just purchase the correct prop and sell your old one to offset the costs.