Proper Prop on used boat

[WanabeBoater]

Hello, I just purchased a 20'1994 Starcraft 2010ss with a 5.7 mercruiser/Alpha 1 stern. I have only test driven the boat for function and was able to get the boat to 55mph with 3 people on board. The current prop is aluminum and one of the fins has a ding on it. The current prop is 14x19. I was wondering if it would be wise to stick with this size prop or would another prop work better for me. I want speed/hole shot and top end. I would just like to maximize my engine all around. The current engine rated horsepower is 230 I believe. I went to a few local marinas to get an owners manual for this boat and they looked as if they didn't know or thought I was out of my mind. Also, can spark plugs for cars work in my marine motor or do I have to purchase marine plugs? Any and all help would be highly appreciated. Thank You.

 

[studlymandingo]

Re: Proper Prop on used boat

Go with a stainless prop for sure. You should be able to go with a 21 or 23 pitch for sure. I'm sure one of the prop gurus will be along to give you the best advice. You will want to check your tach at WOT (wide open throttle) you should be pushing max rpms. If the boat wants to go past max rpms, you'll want a bigger pitch, if you aren't quite getting there, a lower pitch.

 

[WanabeBoater]

Re: Proper Prop on used boat

Studlymandingo, Thanks for your advise..I wish I knew what my operating rpms were. W/O the manuals kinda of makes it tough. But every little bit of info I get...sooner or later my question will be figured out. Thanks again.

 

[QC]

Re: Proper Prop on used boat

There is no way to guess at the correct pitch without knowing your existing WOT RPM today as is. I would say, if you are seeing 55 MPH verified with GPS then you are not far off. If you are getting that speed number from your boat speedo, it is probably wrong. There are very few 20 footers running an honest 55 with a Carbed 5.7, especially of that vintage and with 3 people on board. The latest 5.7 carbed are 250 bhp at the propshaft. I believe you are correct at 230 bhp in 1994.<br /><br />First thing you need to do is to verify max RPM (tach must be correct) at WOT. Load should be typical and trimmed out to the max possible speed. I believe the spec on your vintage was 4400 - 4800 RPM. It is not necessarily true that the high end of that range is the best for top speed. The high end is best for all around use, but most data show that propped toward the bottom is better for top speed, definitely worse for hole shot. Yes, stainless is the way to go . . . need good data first!<br /><br />BTW, Welcome to iboats!!!

 

[DOCD9501]

Re: Proper Prop on used boat

One option for you is to go to your local Merc dealer. Some Merc dealers have a LOAN A PROP program. You leave a copy of your DL and maybe a check and you can borrow props till ya find the one you like. But like the guys said find out your WOT first and remember high rpm go larger low rpm go smaller in pitch. Another thing you might keep in mind seeing you are looking for the best in all worlds is that 4 blades give ya best hole shots, and all round performance, were 3 blade props maximize speed, but to pitch to max speed = slower hole shots. 4 blades are theoretically 2 pitches lower compaired to a = 3 blade. ( 24p 4blade = 26p 3blade) Good luck!

 

[Silvertip]

Re: Proper Prop on used boat

Knowing your maximum RPM consists of two parts. Maximum "recommended" wide open throttle RPM is the maximum RPM the engine manufacturer (GM in this case) says is optimal. The style, diameter and pitch of the prop, the load you normally carry will determine what the engine actually revs to when wide open. Your tach tells you that. 4600 - 4800 RPM is the wide open throttle range for the 5.7 GM engine. If you can't reach that RPM, you are lugging the engine and need a prop with less pitch for that particular load. If the tach shows higher than the recommended range you are over-reving at that load and need a prop with more pitch. There is no free lunch when it comes to hole-shot and top speed. You either have power (hole-shot) or top-speed. One prop can get you one or the other, and its best to be somewhere in between. Otherwise you need two props -- one for water sports and one for top end/cruising.

 

[walleyehed]

Re: Proper Prop on used boat

but most data show that propped toward the bottom is better for top speed,

Not pickin' but I'd like to see the proof on that. We need to know where max torque is produced (RPM) before we can say lower RPM produces more speed.<br />A prop designed for that hull will have much more effect on speed at any RPM by lifting in the correct manner than just dropping the RPM by adding pitch.<br />For example...I ran a 19P michigan wheel SS, then swapped over to a 20P Raker...I picked up 200 RPM and nearly 6 mph. Lesson is blade design has as much if not more to do with speed than RPM does. I can pull back that 200RPM and keep a faster cruise at the same RPM. <br />We prop for max RPM for max engine life at cruise, and if you have the correct set-up, you'll have the speed too.

 

[Silvertip]

Re: Proper Prop on used boat

Didn't say lower RPM could or could not produce more speed. I said if the engine doesn't rev in its recommended RPM range, a prop with less pitch will allow that to happen. The converse is also true. Fact is when people ask for maximum hole shot and maximum top end they can't have it. The fine line is in the request which should probably be stated "good hole shot with maximum speed" or "maximum hole shot with great top speed". Whatever prop one selects, if that happens to permit best top end, you are automatically at maximum hole shot "for that particular prop" but you may not be at maximum hole shot for the engine-boat combination. You can help it a little with tweaks in setup but one needs a different prop and that reverses the situation. You now have maximum possible hole shot, but at the sacrifice of top end. It's all a moot point if tests are not done with the same load and same weather conditions. This is also an I/O -- not much X-dimension to play with. Is this always the case -- certainly not and in the end the owner, after much frustration trying to outsmart the laws of physics, will settle on a prop that provides "acceptable top end" AND "acceptable hole shot". One or the other will be absolute maximum -- but not both at the same time.

 

[QC]

Re: Proper Prop on used boat

UIS,<br /><br />He was reacting to me. I posted it.<br /><br />Kenny,<br /><br />Not pickin' either, I just regurgitate stuff I read, and have learned after 28+ years in the engine business. Most of the I/O data I have seen supports what I posted about the low end of the WOT range. I have zero experience with outboards . . . Usually very marginal difference. I don't agree that you need to know where peak torque is. That is usually 1000+ RPM below WOT range (I/O), so it simply doesn't come into play at top speed. Again, I don't mean to pick, but these forums are about sharing and Torque is one of those things that is misunderstood, especially in marine applications. You have waaaaaay more experience with propellers than I do by a huge margin.<br /><br />BTW, your own test data supports (not confirms) my comment Yes, I know they're different props, but the fastest top speed setups in your tests were consistently among the lower RPM examples.<br /><br />I am not recommending that anybody prop for the low RPM end, as the boat is less flexible, it's just an observation.

 

[walleyehed]

Re: Proper Prop on used boat

QC, good catch on my testing, but also be aware that the more pitch I added, the cylinder head temps AND fuel use both increased, and both results of lugging. In fact, I had a very noted rise in temp well before I got to the bottom of my range<br />That being said, I still think if we prop for the top of the RPM range, we will have lower engine temp and better fuel economy at any throttle setting.<br />Torque plays a bigger role than one may think, and that's why a 4-stroke is slow out of the hole and will never have the mid-range of a 2-stroke, and it was proven with the E-Tec on dual engine installations...a 4-stroke on 1 side, E-Tec on the other..when both were set-up so the handling was managable, they tilted the E-Tec up, and ran only the 4-stroke (2 brands), neither would plane the boat alone, but when the 4-strokes were tilted up, the 2-stroke would plane it out without fail.<br />There is still many variables that can change any of this from 1 rig to another, but you know that already.<br />We have a 1984 20ft wellcraft cuddy with a stock 228/305, and although "recommended" is a max of 4800, we run it 5500-5700, and have for 21 years...we cruise about 4500 with it, but it's set up for the "Family thing". I would never dream of setting it up for under 5000.

 

[QC]

Re: Proper Prop on used boat

Agree, saw the temp stuff too.<br /><br />Also on the torque thing, all of my previous ramblings have noted that if you have two equally rated engines (bhp), theoretically they should achieve the same top speed (propped, and geared properly), the one with higher peak torque will have better hole shot and may plane a boat that otherwise won't. My biggest argument has been with those that believe a 310 bhp carbed big block will be faster at top speed than a 320 MPI 6.2 due to increased torque from the increased displacement (and I bet the 6.2 has more torque anyway, but that is yet another discussion). Also I have been saying (and still strongly believe) that for an application like a big cruiser, if it planes as is, increased low end torque will be meaningless. If you wanna go faster, you gotta have more horsepower, which means more torque at WOT RPM, but definitely not "peak" torque which occurs at a much lower RPM.<br /><br />Funny thing, the answer is often in their question "why don't marine manufacturers provide torque specs?". It is also interesting that even supposed "experts" writing in boating magazines say stuff like "marine applications require more torque than automotive" when exactly the opposite is true. I've even seen writers claim that the new 4 stroke OBs are superior because of more torque, which is bass ackwards as your test data supports . . .<br /><br />BTW, the ETECs are still lighter than their 4 stroke competition and this is yet another reason they will be faster, and all OBs will be faster than equally rated I/Os for the same reason. You da man!

 

[WanabeBoater]

Re: Proper Prop on used boat

Agree, saw the temp stuff too.<br /><br />Also on the torque thing, all of my previous ramblings have noted that if you have two equally rated engines (bhp), theoretically they should achieve the same top speed (propped, and geared properly), the one with higher peak torque will have better hole shot and may plane a boat that otherwise won't. My biggest argument has been with those that believe a 310 bhp carbed big block will be faster at top speed than a 320 MPI 6.2 due to increased torque from the increased displacement (and I bet the 6.2 has more torque anyway, but that is yet another discussion). Also I have been saying (and still strongly believe) that for an application like a big cruiser, if it planes as is, increased low end torque will be meaningless. If you wanna go faster, you gotta have more horsepower, which means more torque at WOT RPM, but definitely not "peak" torque which occurs at a much lower RPM.<br /><br />Funny thing, the answer is often in their question "why don't marine manufacturers provide torque specs?". It is also interesting that even supposed "experts" writing in boating magazines say stuff like "marine applications require more torque than automotive" when exactly the opposite is true. I've even seen writers claim that the new 4 stroke OBs are superior because of more torque, which is bass ackwards as your test data supports . . .<br /><br />BTW, the ETECs are still lighter than their 4 stroke competition and this is yet another reason they will be faster, and all OBs will be faster than equally rated I/Os for the same reason. You da man!