rpm range

[sonny1]

hi all, a question if some one can enlighten me please, my tohatsu 30hp motor has a maximum rpm range somewhere between 5250 and6250 according to the manual, does this mean that two identical motors in the same setup and/or conditions can vary by up to 1000rpm or am i not understanding that particular spec. i am thinking if possible of getting a rev counter set up permanently . i bought one off ebay ,as yet not fitted , but i rather fancy some thing a bit more robust sat next to my garmin, you know the one , a big readable clock face. i thought that given there is a permanent 12v supply this would not be a problem or am i deluding myself ,or maybe something a bit more comprehensive with perhaps some more instruments on it. any ideas to pimp up my boat thanks in anticipation.....terry ..(it just gives me something to occupy my mind,theres not much chance of me using them and even less of understanding them)

[chris moody]

It means you should select a propeller of a diameter and pitch than puts the engine inside this rev range when running at wide open throttle. If the engine cannot make 5250 rpm you are over propped and the engine will be labouring. If you exceed 6250 you are under propped and not loading the engine up enough. It’s a bit like trying to cycle in too low or too high a gear, you will just wear yourself out and get nowhere.

[sonny1]

aaah ,so the manual is telling me that the engine needs to be working within that rpm range to be efficient so a rev counter is a need rather than an accessory. can you suggest anything with regard to the second part of my question re: a rev counter,preferably mounted next to my garmin and a reasonable size so its easily visible/readable. the one i bought from ebay would be difficult for me to see sitting in my armchair ,let alone bouncing about at full throttle. any suggestions gratefully received

[chris moody]

A rev counter isn’t needed after you have done the wide open throttle tests and selected the optimum propeller. Just use the eBay one for prop testing, then remove it.

If you had gears on an outboard a tachometer would be useful, but as we all only have one gear it’s an unnecessary bit of kit. Water depth, direction of travel, and speed are the three most useful bits of information you can have on a boat (in that order of importance in my opinion).

[chris moody]

The engine doesn’t need to be working in that rev range, it just needs to be between those two numbers when at wide open throttle.

[sonny1]

okay ,that puts that one to bed then. thanks chris(moody) for the tips

[harrison]

Personally find the little rev counters really useful, even when not prop testing or running in. Always comforting to know what the actual revs are and the hour meters great for keeping an accurate track of errr hours.

If you want a large analog gauge then probably best to go for a Tohatsu one;
http://www.tohatsu.co.uk/accessories-to ... auges.html

If you’ve already got one of the small induction rev/hour meters can you not mount it on the end of your multi function tiller?

Reckon even I could read it here

PS: @sonny1 Been out much in ya new boat?

[sonny1]

you are a star ian , i need to meet up with you soon . when are you south coast again . im booked in at redcliffe farm for three days 15/16/17 august(poole harbour)if you are at a loose end any of those days. i would be interested to know more about a tachometer suited to my multfunction tiller and will be on the phone early doors tomorrow to laurence to check it out thanks . with regard to :have i been out much: its 3 times on my boat and twice on my friends xs460 . im doing lymington a week on saturday(3rd august) lovely launch there and a good market to keep the wife happy, if youve never done it i can recommend it highly

[harrison]

Hey sonny,
Busy boating up in the frozen north most of the time over the summer, so struggling to get south unfortunately. Will try and attend any forum jaunts though.
Have a great time at Lymington and Poole. Hope to catch up soon.